Sort the conditional statements (#946)

This patch will sort the conditional statements to reduce the conditional branches
by skipping if statements that do not need to be executed depending on the value of `first`.

−       | before   | after    | result
--      | --       | --       | --
Oj.load | 325.594k | 341.356k | 1.048x

### Environment
- Linux
  - Manjaro Linux x86_64
  - Kernel: 6.12.4-1-MANJARO
  - AMD Ryzen 9 8945HS
  - gcc version 14.2.1
  - Ruby 3.4.1

### Code
```ruby
require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
  gem 'oj'
end

json = '{"a":"Alpha","b":true,"c":12345,"d":[true,[false,[-123456789,null],3.9676,["Something else.",false],null]],"e":{"zero":null,"one":1,"two":2,"three":[3],"four":[0,1,2,3,4]},"f":null,"h":{"a":{"b":{"c":{"d":{"e":{"f":{"g":null}}}}}}},"i":[[[[[[[null]]]]]]]}'

Benchmark.ips do |x|
  x.time = 10
  x.report('Oj.load compat') { Oj.load(json, mode: :compat) }
end
```

### Before
```
$ ruby json_load.rb
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
Warming up --------------------------------------
      Oj.load compat    32.352k i/100ms
Calculating -------------------------------------
      Oj.load compat    325.594k (± 1.8%) i/s    (3.07 μs/i) -      3.268M in  10.039265s
```

### After
```
$ ruby json_load.rb
ruby 3.4.1 (2024-12-25 revision 48d4efcb85) +PRISM [x86_64-linux]
Warming up --------------------------------------
      Oj.load compat    34.274k i/100ms
Calculating -------------------------------------
      Oj.load compat    341.356k (± 3.6%) i/s    (2.93 μs/i) -      3.427M in  10.056686s
```

ext/oj/parse.c

@@ -681,26 +681,28 @@ void oj_parse2(ParseInfo pi) {
     pi->cur = pi->json;
     err_init(&pi->err);
     while (1) {
-        if (0 < pi->max_depth && pi->max_depth <= pi->stack.tail - pi->stack.head - 1) {
+        if (RB_UNLIKELY(0 < pi->max_depth && pi->max_depth <= pi->stack.tail - pi->stack.head - 1)) {
             VALUE err_clas = oj_get_json_err_class("NestingError");
 
             oj_set_error_at(pi, err_clas, __FILE__, __LINE__, "Too deeply nested.");
             pi->err_class = err_clas;
             return;
         }
         next_non_white(pi);
-        if (!first && '\0' != *pi->cur) {
-            oj_set_error_at(pi,
-                            oj_parse_error_class,
-                            __FILE__,
-                            __LINE__,
-                            "unexpected characters after the JSON document");
-        }
-
-        // If no tokens are consumed (i.e. empty string), throw a parse error
-        // this is the behavior of JSON.parse in both Ruby and JS.
-        if (No == pi->options.empty_string && 1 == first && '\0' == *pi->cur) {
-            oj_set_error_at(pi, oj_parse_error_class, __FILE__, __LINE__, "unexpected character");
+        if (first) {
+            // If no tokens are consumed (i.e. empty string), throw a parse error
+            // this is the behavior of JSON.parse in both Ruby and JS.
+            if (RB_UNLIKELY('\0' == *pi->cur && No == pi->options.empty_string)) {
+                oj_set_error_at(pi, oj_parse_error_class, __FILE__, __LINE__, "unexpected character");
+            }
+        } else {
+            if (RB_UNLIKELY('\0' != *pi->cur)) {
+                oj_set_error_at(pi,
+                                oj_parse_error_class,
+                                __FILE__,
+                                __LINE__,
+                                "unexpected characters after the JSON document");
+            }
         }
 
         switch (*pi->cur++) {
@@ -761,7 +763,7 @@ void oj_parse2(ParseInfo pi) {
         case '\0': pi->cur--; return;
         default: oj_set_error_at(pi, oj_parse_error_class, __FILE__, __LINE__, "unexpected character"); return;
         }
-        if (err_has(&pi->err)) {
+        if (RB_UNLIKELY(err_has(&pi->err))) {
             return;
         }
         if (stack_empty(&pi->stack)) {