elan: Assert on a possible division by zero in elan_process_frame_linear()

libfprint/drivers/elan.c:249:26: warning: Division by zero
                px = (px - min) * 0xff / (max - min);
                     ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~

libfprint/drivers/elan.c

@@ -243,6 +243,8 @@ static void elan_process_frame_linear(unsigned short *raw_frame,
 			max = raw_frame[i];
 	}
 
+	g_assert(max != min);
+
 	unsigned short px;
 	for (int i = 0; i < frame_size; i++) {
 		px = raw_frame[i];