This reverts commit bf2b7a78.

There are actually two off-by-one errors, one in WS_Assert_Allocated(),
and another in the use of -1 to have WS_Assert_Allocated() compute
the size of the string, including the null terminator.

We don't need the subject with the null terminator in the ov_t
structure (PRIV_TASK data), because we just need it to get backrefs
from the indices in the ov vector returned from pcre.

Also, there's a system in the VTC tests that may not be obvious.
The b*.vtc tests are about regex matching, c*.vtc for backrefs.