tdf#95481 catch out-of-range access in vcl bitmap

blendBitmap24 assumes bitmap is 2 lines high for doing
interpolation. For bitmaps of height 1, really nothing to interpolate
there. Fix cornercase by 'blending' between the very same line instead.

Change-Id: I9b94000aa563e525d0bb2418346ad2c86af26df8
Reviewed-on: https://gerrit.libreoffice.org/19863Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>

vcl/source/outdev/bitmap.cxx

@@ -810,11 +810,13 @@ public:
             const long nMapY  = mpMapY[nY];
             const long nMapFY = mpMapYOffset[nY];
 
-            pLine0 = pSource->GetScanline(nMapY);
-            pLine1 = pSource->GetScanline(nMapY + 1);
+             pLine0 = pSource->GetScanline(nMapY);
+            // tdf#95481 guard nMapY + 1 to be within bounds
+            pLine1 = (nMapY + 1 < pSource->Height()) ? pSource->GetScanline(nMapY + 1) : pLine0;
 
             pLineAlpha0 = pSourceAlpha->GetScanline(nMapY);
-            pLineAlpha1 = pSourceAlpha->GetScanline(nMapY + 1);
+            // tdf#95481 guard nMapY + 1 to be within bounds
+            pLineAlpha1 = (nMapY + 1 < pSourceAlpha->Height()) ? pSourceAlpha->GetScanline(nMapY + 1) : pLineAlpha0;
 
             pDestScanline = pDestination->GetScanline(nY);