smp: fix SIGSEGV for "smp off" during target examine
The gdb subsystem is initialized after the first target examine,
so the field struct target::gdb_service is NULL during examine.
A command "smp off" in the examine event handler causes a SIGSEGV
during OpenOCD startup.
Check for pointer not NULL before dereferencing it.
Change-Id: Id115e28be23a957fef1b97ab66d7273f0ea0dce4
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8216
Tested-by: jenkins