py/mpprint: Add assertion for, and comment about, valid base values.

py/mpprint.c

@@ -202,6 +202,11 @@ STATIC int mp_print_int(const mp_print_t *print, mp_uint_t x, int sgn, int base,
 }
 
 int mp_print_mp_int(const mp_print_t *print, mp_obj_t x, int base, int base_char, int flags, char fill, int width, int prec) {
+    // These are the only values for "base" that are required to be supported by this
+    // function, since Python only allows the user to format integers in these bases.
+    // If needed this function could be generalised to handle other values.
+    assert(base == 2 || base == 8 || base == 10 || base == 16);
+
     if (!MP_OBJ_IS_INT(x)) {
         // This will convert booleans to int, or raise an error for
         // non-integer types.