Get rid of untested sizeof(secp256k1_ge_storage) == 64 code path #1480

This gets rid of an untested code path. Resolves #1352.

secp256k1_ge_storage is a struct with two secp256k1_fe_storage fields.
The C standard allows the compiler to add padding between the fields and
at the end of the struct, but no sane compiler in the end would do this:
The only reason to add padding is to ensure alignment, but such padding
is never necessary between two fields of the same type.

Similarly, secp256k1_fe_storage is a struct with a single array of
uintXX_t. No padding is allowed between array elements. Again, C allows
the compiler to insert padding at the end of the struct, but there's no
absolute reason to do so in this case.

For the uintXX_t itself, this guaranteed to have no padding bits, i.e.,
it's guaranteed to have exactly XX bits.

So I claim that for any existing compiler in the real world,
sizeof(secp256k1_ge_storage) == 64.

This also splits the big "&&" expression into separate expressions. If
we ever see an assertion fail, the error message will tell it precisely
which one failed.