cmd_nand: fix crashing bug in nand read/write

Commit 418396e212 introduced a
bug that causes nand read and nand write to crash in strcmp
due to a null pointer.

Root cause is that strchr(cmd, '.') returns a null pointer when
the input string does not contain a '.'

The strcmp function does not check for null pointers, resulting
in a crash.

Signed-off-by: Steve Sakoman <steve@sakoman.com>
Signed-off-by: Scott Wood <scott@tyr.buserror.net>

common/cmd_nand.c

@@ -617,7 +617,7 @@ int do_nand(cmd_tbl_t * cmdtp, int flag, int argc, char * const argv[])
 
 		s = strchr(cmd, '.');
 
-		if (!strcmp(s, ".raw")) {
+		if (s && !strcmp(s, ".raw")) {
 			raw = 1;
 
 			if (arg_off(argv[3], &dev, &off, &size))