[ruby/json] initialize search.chunk_end to silence a warning about it being potentially uninitialized

ruby/json@73818b80c8

Author: samyron

ext/json/generator/generator.c

@@ -1119,6 +1119,7 @@ static void raw_generate_json_string(FBuffer *buffer, struct generate_json_data
     search.matches_mask = 0;
     search.has_matches = false;
     search.chunk_base = NULL;
+    search.chunk_end = NULL;
 #endif /* HAVE_SIMD */
 
     switch (rb_enc_str_coderange(obj)) {