gdbstub: Fix buffer overflows in gdb_handle_packet()

Some places in gdb_handle_packet() can get an arbitrary length (most
times directly from the client) and either didn't check it at all or
checked against the wrong value, potentially causing buffer overflows.

Cc: qemu-stable@nongnu.org
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>