libmm-glib,location-3gpp: fix string overflow

mm-location-3gpp.c: In function ‘mm_location_3gpp_get_mobile_country_code’:
  mm-location-3gpp.c:139:12: warning: writing 1 byte into a region of size 0 [-Wstringop-overflow=]
    139 |     mcc[4] = '\0';
        |     ~~~~~~~^~~~~~
  mm-location-3gpp.c:132:11: note: at offset 4 to object ‘mcc’ with size 4 declared here
    132 |     gchar mcc[4];
        |           ^~~

libmm-glib/mm-location-3gpp.c

@@ -136,7 +136,7 @@ mm_location_3gpp_get_mobile_country_code (MMLocation3gpp *self)
     if (!self->priv->operator_code)
         return 0;
     memcpy (mcc, self->priv->operator_code, 3);
-    mcc[4] = '\0';
+    mcc[3] = '\0';
     return strtol (mcc, NULL, 10);
 }