Fix shader crash when the comma used in `for` loop as a trailing

servers/rendering/shader_language.cpp

@@ -898,6 +898,13 @@ bool ShaderLanguage::_lookup_next(Token &r_tk) {
 	return false;
 }
 
+ShaderLanguage::Token ShaderLanguage::_peek() {
+	TkPos pre_pos = _get_tkpos();
+	Token tk = _get_token();
+	_set_tkpos(pre_pos);
+	return tk;
+}
+
 String ShaderLanguage::token_debug(const String &p_code) {
 	clear();
 
@@ -8080,6 +8087,11 @@ Error ShaderLanguage::_parse_block(BlockNode *p_block, const FunctionInfo &p_fun
 						_set_error(RTR("The middle expression is expected to be a boolean operator."));
 						return ERR_PARSE_ERROR;
 					}
+					tk = _peek();
+					if (tk.type == TK_SEMICOLON) {
+						_set_error(vformat(RTR("Expected expression, found: '%s'."), get_token_text(tk)));
+						return ERR_PARSE_ERROR;
+					}
 					continue;
 				}
 				if (tk.type != TK_SEMICOLON) {
@@ -8088,6 +8100,11 @@ Error ShaderLanguage::_parse_block(BlockNode *p_block, const FunctionInfo &p_fun
 				}
 			} else if (p_block->block_type == BlockNode::BLOCK_TYPE_FOR_EXPRESSION) {
 				if (tk.type == TK_COMMA) {
+					tk = _peek();
+					if (tk.type == TK_PARENTHESIS_CLOSE) {
+						_set_error(vformat(RTR("Expected expression, found: '%s'."), get_token_text(tk)));
+						return ERR_PARSE_ERROR;
+					}
 					continue;
 				}
 				if (tk.type != TK_PARENTHESIS_CLOSE) {

servers/rendering/shader_language.h

@@ -1037,6 +1037,7 @@ private:
 	Token _make_token(TokenType p_type, const StringName &p_text = StringName());
 	Token _get_token();
 	bool _lookup_next(Token &r_tk);
+	Token _peek();
 
 	ShaderNode *shader = nullptr;