Avoid null target->semihosting references.

The new common semihosting code introduced a bug,
in certain conditions target->semihosting was
used without semihosting being initialised.

The solution was to explicitly test for
target->semihosting before dereferencing it.

Change-Id: I4c83e596140c68fe4ab32e586e51f7e981a40798
Signed-off-by: Liviu Ionescu <ilg@livius.net>
Reviewed-on: http://openocd.zylin.com/4603
Tested-by: jenkins
Reviewed-by: Jonathan Larmour <jifl@eCosCentric.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>

src/target/arm_semihosting.c

@@ -82,6 +82,9 @@ static int post_result(struct target *target)
 {
 	struct arm *arm = target_to_arm(target);
 
+	if (!target->semihosting)
+		return ERROR_FAIL;
+
 	/* REVISIT this looks wrong ... ARM11 and Cortex-A8
 	 * should work this way at least sometimes.
 	 */