Fix crash when server returns non-standard EXPUNGE response

Apparently some servers don't mention "EXPUNGE" in untagged EXPUNGE
response. This wasn't handled correctly due to a typo in response
length check. The code no longer crashes in such case, but will now
print "Unhandled response" warning, which is OK, since we don't
handle results of the EXPUNGE command anyway.

BUG: 364045
FIXED-IN: 16.04.3