Avoid null target->semihosting references.

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>