WS_ReserveSize() does not leave the workspace reserved when the
reservation fails, so WS_Release() must be called for retval > 0 only.

Besides the debug string, it is identical to WS_Reserve() except for

	assert(bytes > 0);

Follow-up varnishcache/varnish-cache#2967

Note: The WS_Reserve() function has not been deprecated as that can
potentially create problems for the build process of VMODs.

(cherry picked from commit 4e333597)