Remove inappropriate @boundscheck

When running julia with `--check-bounds=no`, the package fails to
precompile. The reason is that compiler elides `@boundscheck`s, which in
this case removes buffer reallocation. I believe that's unwanted.

Issue was [reported on
Discourse](https://discourse.julialang.org/t/json3-fails-to-precompile-due-to-embedded-nuls-in-c-strings/128482) by 0samuraiE.

Author: barucden

src/write.jl

@@ -65,7 +65,7 @@ end
 
 macro writechar(chars...)
     block = quote
-        @boundscheck @check($(length(chars)))
+        @check($(length(chars)))
     end
     for c in chars
         push!(block.args, quote