Utility/CppConvert: fix Cython warning (GH-3108)
Utility/CppConvert: fix Cython warning (GH-3108)

When compiling to C++ code and using automatic conversion from Python
str to C++ string, Cython inserts this function template but generates the warning:
warning: string.from_py:15:63: local variable 'length' might be referenced before assignment
scoder/cython,scoder/cython,cython/cython,da-woods/cython,cython/cython,cython/cython,scoder/cython,cython/cython,scoder/cython,da-woods/cython,da-woods/cython,da-woods/cython