packet-amqp.c: Temporary/preliminary fix for Bug #10582 (crash)

This is a temporary/preliminary fix to prevent the buildbot fuzz-test
 failures seen when testing the capture file attached to Bug #10582.

As noted in the bug, a complete fix will require some reworking of
  the amqp dissector.

Change-Id: I2571f1eeb46a7eede04b8d809e87b95f1ff529df
Reviewed-on: https://code.wireshark.org/review/4835
Reviewed-by: Bill Meier <wmeier@newsguy.com>