Add limit with expressions (and null panic workaround)

check.sh

@@ -113,7 +113,7 @@ grep -v "CAST does not support casting from" | # random nulls
 
 # Functions returning sets:
 grep -v "number of columns must be a positive integer literal" |
-grep -v "regex_extract requires a string literal as its first argument" |
+grep -v "regexp_extract requires a string literal as its first argument" |
 grep -v -E "column \".*\" does not exist" | # https://github.com/MaterializeInc/materialize/issues/20533
 grep -v -E "role \".*\" does not exist" |
 grep -v "SET clause does not support casting from interval to integer" |

grammar.cc

@@ -334,9 +334,10 @@ query_spec::query_spec(prod *p, struct scope *s, bool lateral) :
 
   search = bool_expr::factory(this);
 
-  if (d6() > 2) {
+  if (d6() > 1) {
     ostringstream cons;
-    cons << "limit " << d100() + d100();
+    auto expr = value_expr::factory(this, sqltype::get("int"), false);
+    cons << "limit coalesce(" << *expr << ", " << d100() + d100() << ")";
     limit_clause = cons.str();
   }
 }