py/mpz: Fix bug with overflowing C-shift in division routine.

When DIG_SIZE=32, a uint32_t is used to store limbs, and no normalisation
is needed because the MSB is already set, then there will be left and
right shifts (in C) by 32 of a 32-bit variable, leading to undefined
behaviour.  This patch fixes this bug.

tests/basics/int_big_mod.py

@@ -8,3 +8,10 @@ for i in range(11):
         y = delta * (j)# - 5) # TODO reinstate negative number test when % is working with sign correctly
         if y != 0:
             print(x % y)
+
+# these check an edge case on 64-bit machines where two mpz limbs
+# are used and the most significant one has the MSB set
+x = 0x8000000000000000
+print((x + 1) % x)
+x = 0x86c60128feff5330
+print((x + 1) % x)