Modestly improve pgbench's checking for invalid ranges.

The old check against MAX_RANDOM_VALUE is clearly irrelevant since
getrand() no longer calls random().  Instead, check whether min and max
are close enough together to avoid an overflow inside getrand(), as
suggested by Tom Lane.  This is still somewhat silly, because we're
using atoi(), which doesn't check for overflow anyway and (at least on
my system) will cheerfully return 0 when given "4294967296".  But that's
a problem for another commit.

Author: robertmhaas

contrib/pgbench/pgbench.c

@@ -1066,9 +1066,23 @@ doCustom(TState *thread, CState *st, instr_time *conn_time, FILE *logfile)
 			else
 				max = atoi(argv[3]);
 
-			if (max < min || max > MAX_RANDOM_VALUE)
+			if (max < min)
 			{
-				fprintf(stderr, "%s: invalid maximum number %d\n", argv[0], max);
+				fprintf(stderr, "%s: maximum is less than minimum\n", argv[0]);
+				st->ecnt++;
+				return true;
+			}
+
+			/*
+			 * getrand() neeeds to be able to subtract max from min and add
+			 * one the result without overflowing.  Since we know max > min,
+			 * we can detect overflow just by checking for a negative result.
+			 * But we must check both that the subtraction doesn't overflow,
+			 * and that adding one to the result doesn't overflow either.
+			 */
+			if (max - min < 0 || (max - min) + 1 < 0)
+			{
+				fprintf(stderr, "%s: range too large\n", argv[0]);
 				st->ecnt++;
 				return true;
 			}