memory: fix segv on qemu_ram_free(block=0x0)

memory: fix segv on qemu_ram_free(block=0x0)

Since f1060c55bf1377b4, the pointer is directly passed to
qemu_ram_free(). However, on initialization failure, it may be called
with a NULL pointer. Return immediately in this case.

This fixes a SEGV when memory initialization failed, for example
permission denied on open backing store /dev/hugepages, with -object
memory-backend-file,mem-path=/dev/hugepages.

Program received signal SIGSEGV, Segmentation fault.
0x00005555556e67e7 in qemu_ram_free (block=0x0) at /home/elmarco/src/qemu/exec.c:1775

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1459250451-29984-1-git-send-email-marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>