Bug#20783: Valgrind uninitialised warning in test case ctype_uca

Two functions have different ideas of what a string should look like;
one of them reads memory it assumes the other one may have written.
And "if you assume ..."
  
We now use a more defensive variety of the assuming function, this fixes
a warning thrown by the valgrind tool.

sql/item_cmpfunc.cc

@@ -3303,7 +3303,7 @@ longlong Item_func_regex::val_int()
     }
   }
   null_value=0;
-  return my_regexec(&preg,res->c_ptr(),0,(my_regmatch_t*) 0,0) ? 0 : 1;
+  return my_regexec(&preg,res->c_ptr_safe(),0,(my_regmatch_t*) 0,0) ? 0 : 1;
 }