tools: kwbimage: Don't crash when binary file name does not contain '/'

In the case when the file name is specified relative to the current
working directory, it does not contain '/' character and strrchr()
returns NULL.

The following strcmp() function then crashes on NULL pointer
dereference.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>
diff --git a/tools/kwbimage.c b/tools/kwbimage.c
index c65a40e..724459f 100644
--- a/tools/kwbimage.c
+++ b/tools/kwbimage.c
@@ -1277,7 +1277,7 @@
 	if (e) {
 		char *s = strrchr(e->binary.file, '/');
 
-		if (strcmp(s, "/binary.0") == 0)
+		if (s && strcmp(s, "/binary.0") == 0)
 			main_hdr->destaddr = cpu_to_le32(params->addr);
 	}