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

Since f1060c55, 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: NMarc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <1459250451-29984-1-git-send-email-marcandre.lureau@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1773,6 +1773,10 @@ static void reclaim_ramblock(RAMBlock *block)
 
 void qemu_ram_free(RAMBlock *block)
 {
+    if (!block) {
+        return;
+    }
+
     qemu_mutex_lock_ramlist();
     QLIST_REMOVE_RCU(block, next);
     ram_list.mru_block = NULL;