qemu-char: avoid segfault if user lacks of permisson of a given logfile

qemu-char: avoid segfault if user lacks of permisson of a given logfile

Function qemu_chr_alloc returns NULL if it failed to open logfile by any reason,
says no write permission. For backends tty, stdio and msmouse, They need to
check this return value to avoid segfault in this case.

Signed-off-by: Lin Ma <lma@suse.com>
Cc: qemu-stable <qemu-stable@nongnu.org>
Message-Id: <20160914062250.22226-1-lma@suse.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
(cherry picked from commit 71200fb9664c2967a1cdd22b68b0da3a8b2b3eb7)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>