wstring_rvalue_from_python crash on zero length PyObject

[glslang]

Using Python 2.3 (OSX native) and OSX 10.4.7 (powerpc-apple-darwin8-
gcc-4.0.1 (GCC) 4.0.1 (Apple Computer, Inc. build 5341))

Boost version is 1_33_1 and was compiled from source.

When PyObject_Length(intermediate) is zero in 
wstring_rvalue_from_python::extract (when referring to an empty string) a 
crash will occur in PyUnicode_AsWideChar, most probably because 0 is 
then passed to it.

Doing a if( result.empty( ) ) return std::wstring( ) right after the result 
string is constructed seems to solve this and produce correct results
on my code.

[david_abrahams]

Logged In: YES 
user_id=52572

This doesn't come with a test case so I can't verify it. 
Regardless, it's not a Boost.Python bug, but I checked in an
attempted workaround anyway.  Please reopen if this fix
doesn't work for you.