Latest commit introduces undefined behavior in hgcd2.c

[Guido Vranken]

My bignum fuzzer running at OSS-Fuzz came up with this:

hgcd2.c:223:42: runtime error: shift exponent 64 is too large for
64-bit type 'mp_limb_t' (aka 'unsigned long')
#0 0x76a4db in div2 /src/libgmp/mpn/hgcd2.c:223:42
#1 0x769684 in __gmpn_hgcd2 /src/libgmp/mpn/hgcd2.c:372:18
#2 0x74ac55 in __gmpn_gcd /src/libgmp/mpn/gcd.c:200:11
#3 0x73c209 in __gmpz_gcd /src/libgmp/mpz/gcd.c

Introduced in commit https://gmplib.org/repo/gmp/rev/f044264e2fe9