at: break before overflow when receiving messages

Previously this code checked if the buffer was full after writing to it,
which meant that the buffer could overflow.

This checks for an overflow before copying into the buffer and only
copies the data that will fit.

src/at.c

@@ -245,7 +245,14 @@ static gboolean modem_response(gint fd,
      */
     do {
         ret = read(fd, tmp, sizeof(tmp));
+
         if (ret > 0) {
+            /* If we're going to overflow truncate the data we read to fit */
+            if (pos + ret >= sizeof(response)) {
+                g_critical("AT response buffer full, truncating");
+                ret = sizeof(response) - (pos + 1);
+            }
+
             memcpy(&response[pos], tmp, ret);
             pos += ret;
             usleep(10000);