qemu-char: Fix crash when switching consoles

qemu-system-arm (and other system emulations) crashes with SDL when
the user switches consoles (Alt-Ctrl-F4).

We already check for NULL pointers in qemu_chr_fe_ioctl,
qemu_chr_be_can_write and other functions, so do this also
for s->chr_read in qemu_chr_be_write. This fixes the crash.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
