fetch/http.c: fix tcc build failure.

fetch/http.c:1475: warning: function might return no value: 'parse_index'

The code uses abort() before returning anything; return a proper
return value instead.

lib/fetch/http.c

@@ -1470,8 +1470,7 @@ parse_index(struct index_parser *parser, const char *buf, size_t len)
 			return -1;
 		return end_attr + 1 - buf;
 	}
-	/* NOTREACHED */
-	abort();
+	return -1;
 }
 
 struct http_index_cache {