vdi: Fix warning from clang

vdi: Fix warning from clang

ccc-analyzer reports these warnings:

block/vdi.c:704:13: warning: Dereference of null pointer
            bmap[i] = VDI_UNALLOCATED;
            ^
block/vdi.c:702:13: warning: Dereference of null pointer
            bmap[i] = i;
            ^

Moving some code into the if block fixes this.
It also avoids calling function write with 0 bytes of data.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 514f21a5d4613e495adc2e2dd48f18091454efb8)

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>