Disallow factorial of negative numbers

The previous implementation returned 1 for all negative numbers, which
is not sensible under any definition.

Discussion: https://www.postgresql.org/message-id/flat/6ce1df0e-86a3-e544-743a-f357ff663f68%402ndquadrant.com

src/backend/utils/adt/numeric.c

@@ -2946,6 +2946,10 @@ numeric_fac(PG_FUNCTION_ARGS)
 	NumericVar	fact;
 	NumericVar	result;
 
+	if (num < 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
+				 errmsg("factorial of a negative number is undefined")));
 	if (num <= 1)
 	{
 		res = make_result(&const_one);

src/test/regress/expected/numeric.out

@@ -2345,14 +2345,6 @@ SELECT 0!;
 (1 row)
 
 SELECT -4!;
- ?column? 
-----------
-        1
-(1 row)
-
+ERROR:  factorial of a negative number is undefined
 SELECT factorial(-4);
- factorial 
------------
-         1
-(1 row)
-
+ERROR:  factorial of a negative number is undefined