Fix error when reading meminfo

On some systems, there are lines in /proc/meminfo that don't match the regex.
Currently these cause resmon to throw an error and give a blank metric. This
change skips those metrics rather than throwing an error.

lib/Core/Memstat.pm

@@ -131,6 +131,7 @@ sub handler {
         open(MEMINFO, '/proc/meminfo') || die "Unable to read proc: $!\n";
         while (<MEMINFO>) {
             /(\w+)\:\s+(\d+).*/;
+            next unless defined($1);
             $metrics{$1} = [$2, 'L'];
         }
         close(MEMINFO);