Critical fixes after review:

- mutex was unlocked before the end of the critical sesion,
- Portability issue: It's better to use (*alloc)(x) instead of alloc(x),
  if alloc is a function passed as an argument.
- Use {} around if() block, to avoid possible problems with some Windows compilers.

sql/sql_string.cc

@@ -370,7 +370,7 @@ bool String::copy(const char *str, uint32 arg_length,
 
 bool String::set_ascii(const char *str, uint32 arg_length)
 {
-  if (!(str_charset->mbminlen > 1))
+  if (str_charset->mbminlen == 1)
   {
     set(str, arg_length, str_charset);
     return 0;