pool: Cleanup error handling in pool_strdup

Note that `git_pool_strdup` cannot really return any error codes,
 because the pool doesn't set errors on OOM.

 The only place where `giterr_set_oom` is called is in
 `git_pool_strndup`, in a conditional check that is always optimized
 away. `n + 1` cannot be zero if `n` is unsigned because the compiler
 doesn't take wraparound into account.

 This check has been removed altogether because `size_t` is not
 particularly going to overflow.
pool: Correct overflow checks

Ok, scrap the previous commit. This is the right overflow check that
takes care of 64 bit overflow **and** 32-bit overflow, which needs to be
considered because the pool malloc can only allocate 32-bit elements in
one go.
pool: Agh, this test doesn't really apply in 32-bit machines

The size_t is 32-bit already, so it overflows before going into the
function. The `-1` test should handle this gracefully in both cases
anyway.