Fixing #3040 (DeskCalcs textview showing wrong/multiple styles)

* fixed crash in StyleBuffer::SetStyleFromMode() if any of the given pointers
  is NULL (which is perfectly valid)
* BTextView::SetFontAndColor() now syncs the null style before it sets the 
  range, thus making sure that there is only one style if the view is not 
  stylable

git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@30516 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/kits/interface/TextView.cpp

@@ -1619,6 +1619,7 @@ BTextView::SetFontAndColor(int32 startOffset, int32 endOffset,
 		endOffset = textLength;
 
 	// add the style to the style buffer
+	fStyles->SyncNullStyle(startOffset);
 	fStyles->SetStyleRange(startOffset, endOffset, fText->Length(),
 		fontMode, &newFont, color);