qemu-sockets: avoid strlen of NULL pointer

If the user wants to create a chardev of type socket but forgets to give a
host= option, qemu_opt_get returns NULL. This NULL pointer is then fed into
strlen a few lines below without a check which results in a segfault.
This fixes it.

Signed-off-by: Jens Osterkamp <jens@linux.vnet.ibm.com>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
