CDRIVER-4747 use size_t consistently in bson_utf8_validate (#1458)

Author: kevinAlbs

src/libbson/src/bson/bson-utf8.c

@@ -118,8 +118,8 @@ bson_utf8_validate (const char *utf8, /* IN */
    bson_unichar_t c;
    uint8_t first_mask;
    uint8_t seq_length;
-   unsigned i;
-   unsigned j;
+   size_t i;
+   size_t j;
 
    BSON_ASSERT (utf8);