Fix single line comment logic.

It seems reasonable to check that langDef.mSingleLineComment is
non-empty before trying to match it with the current line of code.

TextEditor.cpp

diff --git a/TextEditor.cpp b/TextEditor.cpp
index 1492dcd..97b373b 100644
--- a/TextEditor.cpp
+++ b/TextEditor.cpp
@@ -1924,7 +1924,8 @@
 						auto from = line.begin() + currentCoord.mColumn;

 						auto& startStr = mLanguageDefinition.mCommentStart;

 						auto& singleStartStr = mLanguageDefinition.mSingleLineComment;

-						if (currentCoord.mColumn + singleStartStr.size() <= line.size() &&

+						if (singleStartStr.size() > 0 &&

+							currentCoord.mColumn + singleStartStr.size() <= line.size() &&

 							equals(singleStartStr.begin(), singleStartStr.end(), from, from + singleStartStr.size(), pred))

 							withinSingleLineComment = true;

 						else if (!withinSingleLineComment && currentCoord.mColumn + startStr.size() <= line.size() &&