Resolves: tdf#143615 clamp relative times to 1.0

User input permits zero-length animations, so whenever we calculate
relative position within the animation time frame, the case
mnMinSimpleDuration == 0.0 means: we're already at the end of the
animation, i.e. set relative time to 1.0

Change-Id: I0e8c1e29f47bd9fa16f04115cf52d3a176e13fb0
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/133005
Tested-by: Jenkins
Reviewed-by: Thorsten Behrens <thorsten.behrens@allotropia.de>

slideshow/source/engine/activities/simplecontinuousactivitybase.cxx

@@ -63,9 +63,12 @@ namespace slideshow::internal
             // perform will be called at least mnMinNumberOfTurns
             // times.
 
-            // fraction of time elapsed
+            // fraction of time elapsed (clamp to 1.0 for zero-length
+            // animations)
             const double nFractionElapsedTime(
-                nCurrElapsedTime / mnMinSimpleDuration );
+                mnMinSimpleDuration != 0.0 ?
+                nCurrElapsedTime / mnMinSimpleDuration :
+                1.0 );
 
             // fraction of minimum calls performed
             const double nFractionRequiredCalls(
@@ -115,7 +118,10 @@ namespace slideshow::internal
             // ===============================
 
             const double nCurrElapsedTime( maTimer.getElapsedTime() );
-            double nT( nCurrElapsedTime / mnMinSimpleDuration );
+            // clamp to 1.0 for zero animation duration
+            double nT( mnMinSimpleDuration != 0.0 ?
+                       nCurrElapsedTime / mnMinSimpleDuration :
+                       1.0 );
 
 
             // one of the stop criteria reached?