convert_pystr_to_str crashes on lone surrogates (std::string(NULL) UB)

[devdanzin]

convert_pystr_to_str in util.h:55-59 passes the return value of PyUnicode_AsUTF8 directly to std::string without checking for NULL. PyUnicode_AsUTF8 returns NULL when the string contains lone surrogates (which cannot be encoded as UTF-8). Passing NULL to std::string(const char*) is undefined behavior — crashes immediately.

Reproducer:

import kiwisolver
kiwisolver.Variable("test\ud800name")  # Segmentation fault

Current code (util.h:55-59):

inline bool
convert_pystr_to_str( PyObject* value, std::string& out )
{
    out = PyUnicode_AsUTF8( value );
    return true;
}

Fix:

inline bool
convert_pystr_to_str( PyObject* value, std::string& out )
{
    const char* s = PyUnicode_AsUTF8( value );
    if( !s )
        return false;
    out = s;
    return true;
}

This function is called from Variable_new, Variable_setName, convert_to_strength, and convert_to_relational_op — all reachable from Python.

Found by cext-review-toolkit.