nrf51: remove dereference of null pointer

found by clang 3.4.

Change-Id: Id499b546f65acd7a719498bc97e33b21d1ba565a
Signed-off-by: Spencer Oliver <[email protected]>
Reviewed-on: http://openocd.zylin.com/2119
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <[email protected]>
Reviewed-by: Andrey Smirnov <[email protected]>

src/flash/nor/nrf51.c

@@ -629,7 +629,7 @@ static int nrf51_write_page(struct flash_bank *bank, uint32_t offset, const uint
 	struct flash_sector *sector = nrf51_find_sector_by_address(bank, offset);
 
 	if (!sector)
-		goto error;
+		return ERROR_FLASH_SECTOR_INVALID;
 
 	if (sector->is_protected)
 		goto error;