Fixed tor_vasprintf on systems without vasprintf.

If tor is compiled on a system with neither vasprintf nor _vscprintf,
the fallback implementation exposes a logic flaw which prevents
proper usage of strings longer than 127 characters:

* tor_vsnprintf returns -1 if supplied buffer is not large enough,
  but tor_vasprintf uses this function to retrieve required length
* the result of tor_vsnprintf is not properly checked for negative
  return values

Both aspects together could in theory lead to exposure of uninitialized
stack memory in the resulting string. This requires an invalid format
string or data that exceeds integer limitations.

Fortunately tor is not even able to run with this implementation because
it runs into asserts early on during startup. Also the unit tests fail
during a "make check" run.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>

  [backported to 0.2.9 by nickm]