stlink: check read_reg result

Change-Id: I284824aa6f5eae8f6e910a482e9f7435e649fc0d
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/657
Reviewed-by: Peter Stuge <peter@stuge.se>
Tested-by: jenkins

src/target/stm32_stlink.c

@@ -111,6 +111,8 @@ static int stm32_stlink_load_core_reg_u32(struct target *target,
 		 * it was removed from r1 docs, but still works.
 		 */
 		retval = stlink_if->layout->api->read_reg(stlink_if->fd, 20, value);
+		if (retval != ERROR_OK)
+			return retval;
 
 		switch (num) {
 		case ARMV7M_PRIMASK: