gmpints: remove invalid ModInt return value treatment

Before returning the computed result, ModInt checked whether it was
negative, it flipped the sign. This should (a) never be necessary,
because the preceding computations should always produce something
non-negative, and (b) would be mathematically incorrect.

Instead, we simply return the result untreated, and added an assertion
triggering if we ever see a negative result here (only enabled if
DEBUG_GMP is set).

src/gmpints.c

@@ -1864,12 +1864,10 @@ Obj ModInt ( Obj opL, Obj opR )
   }
 
   /* return the result                                                     */
-  if ( IS_INTNEG(mod) )
-    return NEW_INTPOS(mod);
-  else if ( IS_INTOBJ(mod) && 0 > INT_INTOBJ(mod) )
-    return INTOBJ_INT(-INT_INTOBJ(mod));
-  else
-    return mod;
+#if DEBUG_GMP
+  assert( !IS_NEGATIVE(mod) );
+#endif
+  return mod;
 }