mruby-rational: fix crash in rational_new_f with negative exponent

rational_new_b() expects both arguments to be bigints, but rational_new_f()
was passing an integer value for the numerator when the exponent was negative.
This caused a segfault in mrb_bint_reduce() which called RBIGINT() on the
integer value.

Test case: 5r**-92 (from oss-fuzz)

Co-authored-by: Claude <noreply@anthropic.com>

mrbgems/mruby-rational/src/rational.c

@@ -361,7 +361,7 @@ rational_new_f(mrb_state *mrb, mrb_float f)
       rat_overflow(mrb);
 #else
       mrb_value d = mrb_bint_lshift(mrb, mrb_bint_new_int(mrb, deno), neg_exp);
-      return rational_new_b(mrb, mrb_int_value(mrb, nume), d);
+      return rational_new_b(mrb, mrb_bint_new_int(mrb, nume), d);
 #endif
     }
   }