Fix C-API exception handling issues (lock OOM, signed char -1, exception preservation)

[dynapx]

Title: Fix C-API exception handling issues (lock OOM, signed char -1)

---

Description

Hi maintainers,

This PR is a follow-up to #256 (which fixed unchecked PyUnicode_AsUTF8 returns). It addresses additional C-API exception handling issues identified during the same robustness audit of the _cffi_backend C extension.

---

1. PyThread_allocate_lock failure does not set exception

Location: ffi_obj.c:ffi_init_once

PyThread_allocate_lock() returns NULL on failure but does not set a Python exception. The code currently returns NULL without setting an exception, leading to SystemError.

Fix: Add PyErr_SetString() with a descriptive error message before returning.

-        if (lock == NULL)
+        if (lock == NULL)
+        {
+            PyErr_SetString(PyExc_RuntimeError,
+                    "failed to allocate thread lock for init_once");
             return NULL;
+        }

---

2. ctypedescr_new OOM does not set exception

Location: _cffi_backend.c:new_struct_or_union_type

ctypedescr_new() returns NULL on OOM but does not set an exception. The caller returns NULL without setting an exception.

Fix: Add PyErr_NoMemory().

-    if (td == NULL)
+    if (td == NULL) {
+        PyErr_NoMemory();
         return NULL;
+    }

---

3. Signed char -1 false negative detection

Location: _cffi_backend.c:convert_from_object

_convert_to_char(init) returns -1 on error, but -1 is also a valid signed char value (representing 0xFF). The current code treats both as error, causing valid \xFF input to fail without setting exception.

Fix: Check PyErr_Occurred() to distinguish between error and valid -1.

-            if (res < 0)
+            if (res < 0 && PyErr_Occurred())
                 return -1;

---

Impact

These issues can cause:

• SystemError when PyThread_allocate_lock fails
• SystemError when ctypedescr_new hits OOM
• Rejection of valid \xFF byte values

[mattip]

Please move to draft status until the issue is discussed.