Drop `assert()´ check in subpath parsing

Drop `assert()´ check in subpath parsing

This is a minor refactor to use `for´ loops instead of `while´ ones
when parsing subpaths, and to condition exiting the second loop on
filling the array, rather than on reaching the end of the string. If
there is a bug in the code which causes the array to be too small, this
will simply clip the result rather than try to assign to out‐of‐bounds
memory. If there is a bug in the code which causes the array to be too
big, the program will loop endlessly rather than fail an `assert()´
check. The improvement in the first case is deemed to justify the
slight degradation of behaviour in the second.