hw/ide/ahci: Do not dma_memory_unmap(NULL)

libFuzzer triggered the following assertion:

  cat << EOF | qemu-system-i386 -M pc-q35-5.0 \
    -nographic -monitor none -serial none -qtest stdio
  outl 0xcf8 0x8000fa24
  outl 0xcfc 0xe1068000
  outl 0xcf8 0x8000fa04
  outw 0xcfc 0x7
  outl 0xcf8 0x8000fb20
  write 0xe1068304 0x1 0x21
  write 0xe1068318 0x1 0x21
  write 0xe1068384 0x1 0x21
  write 0xe1068398 0x2 0x21
  EOF
  qemu-system-i386: exec.c:3621: address_space_unmap: Assertion `mr != NULL' failed.
  Aborted (core dumped)

This is because we don't check the return value from dma_memory_map()
which can return NULL, then we call dma_memory_unmap(NULL) which is
illegal. Fix by only unmap if the value is not NULL (and the size is
not the expected one).

Cc: qemu-stable@nongnu.org
Reported-by: Alexander Bulekov <alxndr@bu.edu>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20200718072854.7001-1-f4bug@amsat.org
Fixes: f6ad2e32 ("ahci: add ahci emulation")
BugLink: https://bugs.launchpad.net/qemu/+bug/1884693


Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: John Snow <jsnow@redhat.com>
Signed-off-by: John Snow <jsnow@redhat.com>