set mpFirstBand back to 0 after deleting it

bff + valgrind

==30470== Invalid read of size 8
==30470==    at 0xA02103F: RegionBand::isSingleRectangle() const (regionband.cxx:294)
==30470==    by 0xA01F562: WriteRegion(SvStream&, Region const&) (region.cxx:1640)
==30470==    by 0x9F01FD4: MetaClipRegionAction::Write(SvStream&, ImplMetaWriteData*) (metaact.cxx:2487)
...
==30470==  Address 0x1d175980 is 0 bytes inside a block of size 56 free'd
==30470==    at 0x4A07991: operator delete(void*) (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==30470==    by 0xA020BB1: RegionBand::implReset() (regionband.cxx:87)
==30470==    by 0xA020EF2: RegionBand::load(SvStream&) (regionband.cxx:249)

Change-Id: I2468d43c7e73b3390f355358990f8af30b5f026e
(cherry picked from commit 1ca892488a7a6a9c35fc99c41cd4af788fe0df6e)
Reviewed-on: https://gerrit.libreoffice.org/11570
Reviewed-by: Andras Timar <andras.timar@collabora.com>
Tested-by: Andras Timar <andras.timar@collabora.com>