gdb_server: check for invalid read memory requests

gdb_server: check for invalid read memory requests

This prevents invalid free(NULL) (that crashes OpenOCD on desktop
distros).

With radare2 it's now a bit more useful, memory access works if I seek
to the correct address, can't test further as it apparently lacks thumb2
support.

Change-Id: I6ec32d09fd52dab53ba765d7f7519baa1f55d973
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1853
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>