variant objects have no (declared) properties, so the `get_properties`
handlers returns a pointer to constant storage for efficiency reasons.
This pointer must not be returned from the `get_gc` handler, though;
instead we set up an own `get_gc` handler and return NULL from it, to
signal that there are no properties to collect.

* PHP-7.3:
  Fix #78694: Appending to a variant array causes segfault

* PHP-7.2:
  Fix #78694: Appending to a variant array causes segfault

`write_dimension` object handlers have to be able to handle `NULL`
`offset`s; for now we simply throw an exception instead of following
the `NULL` pointer.

As of PHP 7.4.0, the `get_property_ptr_ptr` handler is mandatory; we
implement it to always return `NULL`, which is equivalent to not
setting the handler in former versions.

We add a portable and faster test case than what has been presented in
the bug ticket.

The php_stream_read() and php_stream_write() functions now return
an ssize_t value, with negative results indicating failure. Functions
like fread() and fwrite() will return false in that case.

As a special case, EWOULDBLOCK and EAGAIN on non-blocking streams
should not be regarded as error conditions, and be reported as
successful zero-length reads/writes instead. The handling of EINTR
remains unclear and is internally inconsistent (e.g. some code-paths
will automatically retry on EINTR, while some won't).

I'm landing this now to make sure the stream wrapper ops API changes
make it into 7.4 -- however, if the user-facing changes turn out to
be problematic we have the option of clamping negative returns to
zero in php_stream_read() and php_stream_write() to restore the
old behavior in a relatively non-intrusive manner.

Although TsHashTable and the according API are supposed to easily make
a HashTable thread-safe, they do not; for instance, there can be race
conditions between finding and updating entries.  We therefore avoid
the usage of TsHashTable in favor of a HashTable with our own mutex
management.

The patch has been provided by krakjoe@php.net; I only did some minor
fixes and tweaks.

We have to free strings returned from php_com_olestring_to_string().