tools lib symbol: Skip non-address kallsyms line

Adding check on failed attempt to parse the address and skip the line
parsing early in that case.

The address can be replaced with '(null)' string in case user don't have
enough permissions, like:

  $ cat /proc/kallsyms
      (null) A irq_stack_union
      (null) A __per_cpu_start
      ...
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20180215122635.24029-2-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/lib/symbol/kallsyms.c

@@ -38,6 +38,10 @@ int kallsyms__parse(const char *filename, void *arg,
 
 		len = hex2u64(line, &start);
 
+		/* Skip the line if we failed to parse the address. */
+		if (!len)
+			continue;
+
 		len++;
 		if (len + 2 >= line_len)
 			continue;