Avoid consecutive return statements

This change keeps static checkers happy.

This change originally came from Heimdal's bundled copy of
libtommath.

Signed-off-by: Ken Dreyer <ktdreyer@ktdreyer.com>

bn_mp_invmod.c

@@ -32,9 +32,9 @@ int mp_invmod (mp_int * a, mp_int * b, mp_int * c)
 
 #ifdef BN_MP_INVMOD_SLOW_C
   return mp_invmod_slow(a, b, c);
-#endif
-
+#else
   return MP_VAL;
+#endif
 }
 #endif