Use mem_free() instead of xfree() in check_time().

xfree() should never be used directly and only be called by one of the
mem_*() wrappers.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/utils.c

@@ -594,7 +594,7 @@ check_time (char *string)
   else if (strlen(s) < 4 && is_all_digit(s) && atoi(s) > 0)
     ret = 2;
 
-  xfree(s);
+  mem_free(s);
   return ret;
 }