Fix out-of-bounds buffer read in Scintilla MMIXAL lexer

Fix off-by-1 error in the loop condition.

Apply our own minimal fix because Upstream is aware of the bug (see
https://sourceforge.net/p/scintilla/bugs/2019/) but refuses to fix it
there.

Closes #18440.

src/stc/scintilla/lexers/LexMMIXAL.cxx

@@ -103,7 +103,7 @@ static void ColouriseMMIXALDoc(Sci_PositionU startPos, Sci_Position length, int
 				char s[100];
 				sc.GetCurrent(s, sizeof(s));
 				if (*s == ':') {	// ignore base prefix for match
-					for (size_t i = 0; i != sizeof(s); ++i) {
+					for (size_t i = 0; i != sizeof(s)-1; ++i) {
 						*(s+i) = *(s+i+1);
 					}
 				}