render: silence a couple LLVM static analyzer warnings
render: silence a couple LLVM static analyzer warnings

The while() checked the pointer for nullness, so the analyzer assumed it could
potentially be null, and thus warned on the reference to it later.

Using a do/while instead means we're only checking for subsequent linked-list
entries, which was the intent here anyway, and avoids the warning.

libass/libass,Akemi/libass,xbmc/libass,Akemi/libass,MrSmile/libass,astiob/libass,ShiftMediaProject/libass,Akemi/libass,libass/libass,ShiftMediaProject/libass,astiob/libass,xbmc/libass,MrSmile/libass