flash/nor/fm4,tms470: fix clang static analyzer warnings

Change-Id: I18c1501918d40453fea6aeeb6f035e46d41fc524
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5366
Tested-by: jenkins
Reviewed-by: Oleksij Rempel <linux@rempel-privat.de>

src/flash/nor/tms470.c

@@ -709,6 +709,7 @@ static int tms470_erase_sector(struct flash_bank *bank, int sector)
 	 * Select one or more bits in FMBSEA or FMBSEB to disable Level 1
 	 * protection for the particular sector to be erased/written.
 	 */
+	assert(sector >= 0);
 	if (sector < 16) {
 		target_read_u32(target, 0xFFE88008, &fmbsea);
 		target_write_u32(target, 0xFFE88008, fmbsea | (1 << sector));