pmap: Fix detail parsing on long mapping lines

pmap: Fix detail parsing on long mapping lines

If the mapping descriptor is longer than 128 chars, the last parsed
character won't be a newline even if the current buffer contains it a
bit further than that. The current code always interprets it as a short
fgets() read instead, and thus keeps calling fgets() until it gets a
newline, dropping valid lines and failing with the following error:

pmap: ERROR: inconsistent detail field in smaps file, line:
 Rss:                 212 kB