gdb_server: fix invalid free

gdb_server: fix invalid free

`gdb_service_free` calls `free(gdb_port_next)`, so this needs to be an
allocated string.  Otherwise we trip up detectors like Android's tagged
pointers.

Change-Id: Ib08ea55a38af4e15c4fbae95f10db0e3684ae1af
Signed-off-by: Jim Paris <jimparis@meta.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8768
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins