expansionOfs cannot be negative

lexerState->expansionOfs is always either set to 0, or updated by
adding a positive quantity:

    if (distance > lexerState->expansions->distance) {
        lexerState->expansionOfs += distance - lexerState->expansions->distance;
        ...
    }

so it will always be positive or zero.

src/asm/lexer.c

@@ -364,7 +364,7 @@ struct LexerState {
 	size_t macroArgScanDistance; /* Max distance already scanned for macro args */
 	bool expandStrings;
 	struct Expansion *expansions;
-	size_t expansionOfs; /* Offset into the current top-level expansion (negative = before) */
+	size_t expansionOfs; /* Offset into the current top-level expansion */
 };
 
 struct LexerState *lexerState = NULL;