locale.c: Make a mask be 1, 2, or 3.

This makes the two bits that ever get set in this mask to be positions 0
and 1.

locale.c

@@ -5455,12 +5455,13 @@ S_my_localeconv(pTHX_ const int item)
 
     /* This is a mask, with one bit to tell S_populate_hash_from_localeconv to
      * populate the NUMERIC items; another bit for the MONETARY ones.  This way
-     * it can choose which (or both) to populate from */
+     * it can choose which (or both) to populate from. */
     U32 index_bits = 0;
 
     /* This converts from a locale index to its bit position in the above mask.
      * */
-#  define INDEX_TO_BIT(i)  (1 << (i))
+#  define INDEX_TO_BIT(i)  (1 << ((i) - LC_NUMERIC_INDEX_))
+    STATIC_ASSERT_STMT(LC_MONETARY_INDEX_ == LC_NUMERIC_INDEX_ + 1);
 
     /* The two categories can have disparate locales.  Initialize them to C and
      * override later whichever one(s) we pay attention to */