Once Dispose() has been called, scrollbars will be NULL, so we need to
check against this in code that might run after Dispose().

Fixes scary warnings on certain widget destruction scenarios.

Closes #993.

X-Scintilla-Bug-URL: https://sourceforge.net/p/scintilla/bugs/1873/
X-Scintilla-Commit-ID: 4e67e6ee45732a048907ef1577cf1efc25729313