comedi: ni_routing: tools: Check when the file could not be opened

- After fopen check NULL before using the file pointer use

Signed-off-by: Ruffalo Lavoisier <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/comedi/drivers/ni_routing/tools/convert_c_to_py.c

···
 {
 	FILE *fp = fopen("ni_values.py", "w");
 
+	if (fp == NULL) {
+		fprintf(stderr, "Could not open file!");
+		return -1;
+	}
+
 	/* write route register values */
 	fprintf(fp, "ni_route_values = {\n");
 	for (int i = 0; ni_all_route_values[i]; ++i)