SQLite Forum: lemon cannot shift-reduce errors

@drh

This seems to fix it for me:
```
--- ../sqlite/tool/lempar.c	2021-08-17 13:36:38.275642753 -0700
+++ lempar.c	2021-08-18 12:58:41.840112123 -0700
@@ -986,6 +986,10 @@
                         yypParser->yytos->stateno,
                         YYERRORSYMBOL)) > YY_MAX_SHIFTREDUCE
         ){
+          if (yyact >= YY_MIN_REDUCE && yyRuleInfoNRhs[yyact-YY_MIN_REDUCE] < 0) {
+            yyact -= YY_MIN_REDUCE - YY_MIN_SHIFTREDUCE;
+            break;
+          }
           yy_pop_parser_stack(yypParser);
         }
         if( yypParser->yytos <= yypParser->yystack || yymajor==0 ){
```