tdf#120703 PVS: dereference before nullptr check

Use a function to consistently check if gImpl is valid before assigning last
exception message.

V595 The 'gImpl' pointer was utilized before it was verified against nullptr.
     Check lines: 2673, 2683.

Change-Id: I18fc0d632c84f37b05e9d1c3c83689e6eaa6f66c
Reviewed-on: https://gerrit.libreoffice.org/71488
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>