megasas: do not call pci_dma_unmap after having freed the frame once

Commit 8cc4678 ("megasas: remove useless check for cmd->frame", 2016-07-17) was
wrong because I trusted Coverity too much.  It turns out that there _is_ a
path through which cmd->frame can become NULL.  After megasas_handle_frame's
switch (md->frame->header.frame_cmd), megasas_init_firmware can be called.
From there, megasas_reset_frames will call megasas_unmap_frame which resets
cmd->frame = NULL.

However, there is another bug to fix in there, because megasas_unmap_frame
is called again after setting the command status.  In this case QEMU should
not do anything, instead it calls pci_dma_unmap again.  Harmless, but
better fix it.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
