fastboot: check argc before using argv

"fastboot -w" would segfault because *argv was invalid when no
non-option arguments were specified.  Check argc > 0 before
dereferencing argv.

Change-Id: I822a799e6a38e4e5c0a4eca48c6343b8a08a6185
diff --git a/fastboot/fastboot.c b/fastboot/fastboot.c
index ff99173..5954444 100644
--- a/fastboot/fastboot.c
+++ b/fastboot/fastboot.c
@@ -825,7 +825,7 @@
         return 1;
     }
 
-    if (!strcmp(*argv, "devices")) {
+    if (argc > 0 && !strcmp(*argv, "devices")) {
         skip(1);
         list_devices();
         return 0;