gdb_server: fix 'null' pointer passed as 'nonnull' parameter

gdb_server: fix 'null' pointer passed as 'nonnull' parameter

The null pointer used as second parameter to gdb_put_packet() is
passed as second parameter to the memcpy() in line 408 of
gdb_put_packet_inner(). In this case memcpy() does not segfault
because also the parameter length is zero. Nevertheless, the
prototype of memcpy() requires a nonnull pointer.

Fixed by passing an empty string in place of the null pointer.

Issue highlighted by clang 7.0.0 with warning message:
"Null pointer passed as an argument to a 'nonnull' parameter"

Change-Id: Ib3dde95d76fcc5fb245ee2d6669e5535d0e0b127
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4946
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>