fix stringop-overflow warning of GCC (#201)

When using a modern GCC to compile c-ares, there is a stringop-overflow warning. 
This patch simply silences the false-positive warning, there is no actual code flaw.

Bug: https://github.com/c-ares/c-ares/pull/201
Fixed By: Andi Schnebinger @Iniesta8

ares_parse_ptr_reply.c

@@ -52,6 +52,7 @@ int ares_parse_ptr_reply(const unsigned char *abuf, int alen, const void *addr,
   int aliascnt = 0;
   int alias_alloc = 8;
   char ** aliases;
+  size_t rr_data_len;
 
   /* Set *host to NULL for all failure cases. */
   *host = NULL;
@@ -124,14 +125,15 @@ int ares_parse_ptr_reply(const unsigned char *abuf, int alen, const void *addr,
           if (hostname)
             ares_free(hostname);
           hostname = rr_data;
-          aliases[aliascnt] = ares_malloc((strlen(rr_data)+1) * sizeof(char));
+          rr_data_len = strlen(rr_data)+1;
+          aliases[aliascnt] = ares_malloc(rr_data_len * sizeof(char));
           if (!aliases[aliascnt])
             {
               ares_free(rr_name);
               status = ARES_ENOMEM;
               break;
             }
-          strncpy(aliases[aliascnt], rr_data, strlen(rr_data)+1);
+          strncpy(aliases[aliascnt], rr_data, rr_data_len);
           aliascnt++;
           if (aliascnt >= alias_alloc) {
             char **ptr;