Reduce the gate on get_random_gnc_numeric.

Fixing Bug 779217 increased the maximum denominator by 10, and that led
to overflows when converting large numbers' denominators from 100 to the
new max.

Author: jralls

src/engine/test-core/test-engine-stuff.c

@@ -474,7 +474,7 @@ get_random_gnc_numeric(void)
      * The loop is to "make sure" we get there.  We might
      * want to make this dependent on "deno" in the future.
      */
-    numer = get_random_gint64 () % (2ULL << 44);
+    numer = get_random_gint64 () % (2ULL << 40);
     if (0 == numer) numer = 1;
     /* Make sure we have a non-zero denominator */
     if (0 == deno) deno = 1;