tools/ccan_depends: Don't crash when no dependencies are found

Running 'tools/ccan_depends --compile ccan/crypto' without this
patch will demonstrate the crash.
Signed-off-by: Jon Griffiths <jon_p_griffiths@yahoo.com>

tools/ccan_depends.c

@@ -46,6 +46,7 @@ int main(int argc, char *argv[])
 	else
 		deps = get_safe_ccan_deps(NULL, argv[1], style, recurse);
 
+	if (deps)
 		for (i = 0; deps[i]; i++)
 			if (strstarts(deps[i], "ccan/") == ccan)
 				printf("%s\n", deps[i]);