This is another crash caused by the rfc2047 decoding bug fixed in the
second prior commit.

In this case, an empty header line followed by a header line starting
with ":", would result in t==end.

The mutt_substrdup() further below would go very badly at that point,
with t >= end+1.  This could result in either a memcpy onto NULL or a
huge malloc call.

Thanks to Chenyuan Mi (@morningbread) for giving a working example
draft message of the rfc2047 decoding flaw.  This allowed me, with
further testing, to discover this additional crash bug.