qemu_tpm: Don't crash if qemuTPMPcrBankBitmapToStr(NULL)

qemu_tpm: Don't crash if qemuTPMPcrBankBitmapToStr(NULL)

Historically, the tpm->data.emulator.activePcrBanks member was an
unsigned int but since it was used as a bitmap it was converted
to virBitmap type instead. Now, the virBitmap is allocated inside
of virDomainTPMDefParseXML() but only if <activePcrBanks/> was
found with at last one child element. Otherwise it stays NULL.

Fast forward to starting a domain with TPM 2.0 and no
<activePcrBanks/> configured. Eventually,
qemuTPMEmulatorBuildCommand() is called, which subsequently calls
qemuTPMEmulatorReconfigure() and finally
qemuTPMPcrBankBitmapToStr() passing the NULL value. Before
rewrite to virBitmap this function would return NULL for empty
activePcrBanks but now, well, now it crashes.

Fixes: 52c7c31c8038aa31d502f59a40e4fb4ba9f61113
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>