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

tools/perf/util/header.c

@@ -930,7 +930,7 @@ static int write_topo_node(int fd, int node)
 		/* skip over invalid lines */
 		if (!strchr(buf, ':'))
 			continue;
-		if (sscanf(buf, "%*s %*d %s %"PRIu64, field, &mem) != 2)
+		if (sscanf(buf, "%*s %*d %31s %"PRIu64, field, &mem) != 2)
 			goto done;
 		if (!strcmp(field, "MemTotal:"))
 			mem_total = mem;