tablefunc: Reject negative number of tuples passed to normal_rand()

The function converted the first argument i.e. the number of tuples to
return into an unsigned integer which turns out to be huge number when
a negative value is passed.  This causes the function to take much
longer time to execute.  Instead, reject a negative value.

(If someone really wants to generate many more result rows, they
should consider adding a bigint or numeric variant.)

While at it, improve SQL test to test the number of tuples returned by
this function.

Author: Ashutosh Bapat <ashutosh.bapat@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/CAG-ACPW3PUUmSnM6cLa9Rw4BEC5cEMKjX8Gogc8gvQcT3cYA1A@mail.gmail.com

contrib/tablefunc/expected/tablefunc.out

@@ -3,12 +3,15 @@ CREATE EXTENSION tablefunc;
 -- normal_rand()
 -- no easy way to do this for regression testing
 --
-SELECT avg(normal_rand)::int FROM normal_rand(100, 250, 0.2);
- avg 
------
- 250
+SELECT avg(normal_rand)::int, count(*) FROM normal_rand(100, 250, 0.2);
+ avg | count 
+-----+-------
+ 250 |   100
 (1 row)
 
+-- negative number of tuples
+SELECT avg(normal_rand)::int, count(*) FROM normal_rand(-1, 250, 0.2);
+ERROR:  number of rows cannot be negative
 --
 -- crosstab()
 --