tools: mkimage: Print human readable error when -d is not specified

When asking mkimage to create a new image file and option -d is not
specified then mkimage show human unfriendly error message:

  mkimage: Can't open (null): Bad address

Without debugger it is hard to debug what is the issue. Function open() is
being called with file name set to NULL. So add a check for this and if it
happens then show human readable message that option -d was not specified.

Signed-off-by: Pali Rohár <pali@kernel.org>

tools/mkimage.c

@@ -600,6 +600,11 @@ int main(int argc, char **argv)
 	}
 
 	if ((params.type != IH_TYPE_MULTI) && (params.type != IH_TYPE_SCRIPT)) {
+		if (!params.datafile) {
+			fprintf(stderr, "%s: Option -d with image data file was not specified\n",
+				params.cmdname);
+			exit(EXIT_FAILURE);
+		}
 		dfd = open(params.datafile, O_RDONLY | O_BINARY);
 		if (dfd < 0) {
 			fprintf(stderr, "%s: Can't open %s: %s\n",