Fix the label search routine in geom_map to not trip up on '\0' bytes.

Fix the label search routine in geom_map to not trip up on '\0' bytes.

* Just do the buf check early and fail out
* If the offset being searched is:

00110000  00 b5 7e 45 61 e2 76 d3  c1 78 dd 15 95 cd 1f f1  |..~Ea.v..x......|

.. and the match string is '.!/bin/sh'

.. then it'll set the match string[0] to '\0', do a strncmp() against
the read buffer, find it's matching two zero-length strings, and think
that's where to start.

MFC after:	2 weeks