pmap: avoid depending on an uninitialised value.

pmap: avoid depending on an uninitialised value.

It's possible for the first entry that this reads from /proc/*/smaps to
start with a map_desc-less line like this:

7fa71b77d000-7fa71c288000 rw-p 00000000 00:00 0

in which case sscanf will only read up to inode, and it won't set c; the
code below would then incorrectly discard the next ("Size:") line.

(With GCC 4.9.0, this bug causes the "pmap extra extended output" test
to fail for me.)