Table lookups return copy of non-const &default vals (addresses #981).

This prevents unintentional modifications to the &default value itself.

src/Val.cc

@@ -1749,7 +1749,7 @@ Val* TableVal::Default(Val* index)
 
 	if ( def_val->Type()->Tag() != TYPE_FUNC ||
 	     same_type(def_val->Type(), Type()->YieldType()) )
-		return def_val->Ref();
+		return def_attr->AttrExpr()->IsConst() ? def_val->Ref() : def_val->Clone();
 
 	const Func* f = def_val->AsFunc();
 	val_list* vl = new val_list();

testing/btest/Baseline/language.table-default-record/out

@@ -0,0 +1,7 @@
+0
+0
+0
+0
+{
+
+}

testing/btest/language/table-default-record.bro

@@ -0,0 +1,24 @@
+# @TEST-EXEC: bro -b %INPUT >out
+# @TEST-EXEC: btest-diff out
+
+type Foo: record {
+	x: count &default=0;
+};
+
+global foo: table[count] of Foo = {} &default=[];
+
+# returns the &default value as usual
+print(foo[0]$x);
+print(foo[1]$x);
+
+# these are essentially no-ops since a copy of the &default value is returned
+# by the lookup
+foo[0]$x = 0;
+foo[1]$x = 1;
+
+# the &default value isn't modified
+print(foo[0]$x);
+print(foo[1]$x);
+
+# table membership isn't modified
+print(foo);