dumpcap: fix buffer overflow on packets larger than 2048 bytes

dumpcap: fix buffer overflow on packets larger than 2048 bytes

When the current capture buffer is too small, it must be increased
before attempting to read the next data packet.

Fix developed by Mikael Kanstrup (and Guy), I added comments such that
the next reader does not have to guess whether "incl_len" is
accidentally used for reading from the buffer (it is not).

Change-Id: I980bd21ac79601a34d57ffc99a34bfb54c297ac0
Fixes: v2.5.0rc0-28-gd0865fd619 ("Allow bigger snapshot lengths for D-Bus captures.")
Bug: 13852
Reviewed-on: https://code.wireshark.org/review/22464
Petri-Dish: Peter Wu <peter@lekensteyn.nl>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Guy Harris <guy@alum.mit.edu>