Simplify rand_str()function

unittest/libmariadb/bulk1.c

@@ -26,8 +26,7 @@ char *rand_str(size_t length) {
     char *dest= (char *)malloc(length+1);
     char *p= dest;
     while (length-- > 0) {
-        size_t index =  (size_t)((double)rand() / RAND_MAX * (sizeof charset - 1));
-        *dest++ = charset[index];
+        *dest++ = charset[rand() % sizeof(charset)];
     }
     *dest = '\0';
     return p;