perf tools: Ensure sscanf does not overrun the "mem" field

Make the parsing robust.

(perf has some other assumptions that BUFSIZE <= MAX_PATH which are
 not touched here)

Reported-by: Jackie Chang
Signed-off-by: Alan Cox <alan@linux.intel.com>
Cc: Alan Cox <gnomes@lxorguk.ukuu.org.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/n/tip-g2uoiwbrpiimb63rx32qv8ne@git.kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>