load_obj_dict unchecked PyDict_New + fontext copy-paste bug + unchecked PyModule_AddObject for UserKeyError

[devdanzin]

Three smaller bugs across different files:

1. load_obj_dict unchecked PyDict_New corrupts dict pointer (signaling.cpp:98)

When forcecreate is true and *dict is NULL, PyDict_New() is called but its return value is not checked. On OOM, NULL is stored into *dict and the function returns true (success). The caller's object now has a corrupted __dict__ pointer.

Fix:

if( forcecreate && !*dict )
{
    *dict = PyDict_New();
    if( !*dict )
        return false;
}

2. Copy-paste bug in fontext.cpp:341 — !PyFontCaps instead of !PyFontStretch

After creating PyFontStretch, the code checks PyFontCaps (the previous variable):

cppy::ptr PyFontStretch( new_enum_class( "FontStretch" ) );
if( !PyFontCaps )   // BUG: should be !PyFontStretch
{
    return -1;
}

If new_enum_class("FontStretch") fails, NULL propagates to later use. Currently latent because creation succeeds, but a real bug if the allocation ever fails.

Also, the slot array is named Nonlocals_Type_slots (copy-paste from dynamicscope.cpp) instead of Font_Type_slots (line 239).

3. Unchecked PyModule_AddObject for UserKeyError (dynamicscope.cpp:927)

The only PyModule_AddObject call in the codebase that doesn't follow the cppy::ptr + check + release() pattern. Return value ignored; on failure the error is silently swallowed.

Fix: use PyModule_AddObjectRef (available since 3.10):

if( PyModule_AddObjectRef( mod, "UserKeyError", UserKeyError ) < 0 )
    return -1;

Found by cext-review-toolkit.