Fix Scintilla crash (#19833)
Fixes #19729
### Summary of the issue:
NVDA occasionally causes Notepad++ (and likely other Scintilla-based
editors) to crash.
### Description of user facing changes:
The crash should no longer occur.
### Description of developer facing changes:
None
### Description of development approach:
Return early if `end < start` in `ScintillaTextInfo._getTextRange`.
