the only call to the ImplFontMetricData ctor is from the ctor of
LogicalFontInstance. LogicalFontInstance is only created via
PhysicalFontFace::CreateFontInstance which is only
from ImplFontCache::GetFontInstance.

The typical call to ImplFontCache::GetFontInstance(..., FontSelectPattern&)
is from the other GetFontInstance which create a new FontSelectPatternAttributes
so rFontSelData.mpFontInstance won't get set in that case

The other case is from the edge-case GetGlyphFallbackFont where mpFontInstance
will be set. I'm reasonably confident that we don't need actually have this
mpFontInstance use, but if it turns out we do then it could be restored to
GetGlyphFallbackFont after it ctors the its LogicalFontInstance return

Change-Id: I77404876b55bceced951d78cb52bc9cb829fc904
Reviewed-on: https://gerrit.libreoffice.org/59371
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>