Update fuzz.c for mg_commalist -> mg_span

Author: scaprile

test/fuzz.c

@@ -75,7 +75,7 @@ int LLVMFuzzerTestOneInput(const uint8_t *data, size_t size) {
   mg_globmatch((char *) data, size, (char *) data, size);
 
   struct mg_str k, v, s = mg_str_n((char *) data, size);
-  while (mg_commalist(&s, &k, &v)) k.len = v.len = 0;
+  while (mg_span(s, &k, &v, ',')) k.len = v.len = 0;
 
   int n;
   mg_json_get(mg_str_n((char *) data, size), "$", &n);