Generic/IncrementDecrementSpacing: fix post-decrement error message

This commit fixes the post-decrement error message when there is one or
more newlines between the variable and the post-decrement operator.

The wrong variable was being used to check whether the post-decrement
and the variable were not on the same line, resulting in an incorrect
error message. The error message should explicitly say that a newline
was found.

Before this commit the error message was:

```
Expected no spaces between $i and the decrement operator; 0 found
```

Now it is:

```
Expected no spaces between the decrement operator and $i; newline found
```

(`newline` instead of `0`)

This commit also includes a test that exercises the modified line.

src/Standards/Generic/Sniffs/WhiteSpace/IncrementDecrementSpacingSniff.php

@@ -135,7 +135,7 @@ public function process(File $phpcsFile, $stackPtr)
                 $fixable = false;
                 $spaces  = 'comment';
             } else {
-                if ($tokens[$stackPtr]['line'] !== $tokens[$nextNonEmpty]['line']) {
+                if ($tokens[$stackPtr]['line'] !== $tokens[$prevNonEmpty]['line']) {
                     $spaces = 'newline';
                 } else {
                     $spaces = $tokens[($stackPtr - 1)]['length'];

src/Standards/Generic/Tests/WhiteSpace/IncrementDecrementSpacingUnitTest.inc

@@ -35,3 +35,6 @@ $obj->prop['key'] ++;
 
 --ClassName::$prop;
 -- ClassName::$prop;
+
+getObject()->count
+++;

src/Standards/Generic/Tests/WhiteSpace/IncrementDecrementSpacingUnitTest.inc.fixed

@@ -34,3 +34,5 @@ $obj->prop['key']++;
 
 --ClassName::$prop;
 --ClassName::$prop;
+
+getObject()->count++;

src/Standards/Generic/Tests/WhiteSpace/IncrementDecrementSpacingUnitTest.php

@@ -53,6 +53,7 @@ public function getErrorList($testFile='')
             $errors[31] = 1;
             $errors[34] = 1;
             $errors[37] = 1;
+            $errors[40] = 1;
 
             return $errors;