Robustify GraphicDescriptor::ImpDetect* reading SvStream.
Robustify GraphicDescriptor::ImpDetect* reading SvStream.

The >>operator of SvStream doesn't initialize a variable if the stream
is faulty. So initialize the variables before usage to prevent accidental
matches. Also explicitly check stream status in while loop to prevent
looping on eof or error.

Change-Id: I21c0acc1c9217b4523c593cbcef5718f4cdc7597
Reviewed-on: https://gerrit.libreoffice.org/4469
Reviewed-by: Michael Stahl <eef8671db506f9731c0438a7613853b6391ca841@redhat.com>
Tested-by: Michael Stahl <eef8671db506f9731c0438a7613853b6391ca841@redhat.com>

JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core,JurassicWordExcel/core