jscript: Make 0 an invalid code offset.

dlls/jscript/engine.c

@@ -2614,7 +2614,7 @@ HRESULT exec_source(exec_ctx_t *ctx, parser_ctx_t *parser, source_elements_t *so
     ctx->parser = parser;
 
     if(source->statement) {
-        if(source->instr_off == -1) {
+        if(!source->instr_off) {
             hres = compile_subscript_stat(ctx->parser, source->statement, from_eval, &source->instr_off);
             if(FAILED(hres) && is_jscript_error(hres))
                 hres = throw_syntax_error(script, ei, hres, NULL);

dlls/jscript/parser.y

@@ -1342,7 +1342,7 @@ static void *new_expression(parser_ctx_t *ctx, expression_type_t type, size_t si
     expression_t *ret = parser_alloc(ctx, size ? size : sizeof(*ret));
 
     ret->type = type;
-    ret->instr_off = -1;
+    ret->instr_off = 0;
 
     return ret;
 }
@@ -1470,7 +1470,7 @@ static source_elements_t *new_source_elements(parser_ctx_t *ctx)
     source_elements_t *ret = parser_alloc(ctx, sizeof(source_elements_t));
 
     memset(ret, 0, sizeof(*ret));
-    ret->instr_off = -1;
+    ret->instr_off = 0;
 
     return ret;
 }