Fix SIGSEGV on malformed Text field

C++ helper c_reraise_kj_exception() (capnp/helpers/capabilityHelper.cpp)
unconditionally dereferences the PyObject* returned by wrap_kj_exception_for_reraise()
(capnp/lib/capnp.pyx). For a specific class of malformed input -- a Cap'n Proto Text
field whose NUL terminator is corrupt -- the wrapper returns NULL, and the subsequent
"obj->ob_type" access dereferences NULL (offset 0x8) inside the C extension, producing
a deterministic, UNCATCHABLE SIGSEGV. libcapnp itself detects the corruption correctly
and would raise a catchable KjException for the sibling code path; only this reraise
helper crashes.

The malformed bytes reach the crash through the documented public API
Type.from_bytes(...) + lazy field access -- exactly how pycapnp consumers deserialize
untrusted Cap'n Proto messages received over the network / RPC / from files. A single
flipped byte in an attacker-controlled message takes down the consuming process; the
crash cannot be caught with try/except, so no graceful degradation is possible.

capnp/helpers/capabilityHelper.cpp

@@ -16,6 +16,9 @@ void c_reraise_kj_exception() {
   }
   catch (kj::Exception& exn) {
     auto obj = wrap_kj_exception_for_reraise(exn);
+    if (obj == nullptr) {
+      return;
+    }
     PyErr_SetObject((PyObject*)obj->ob_type, obj);
     Py_DECREF(obj);
   }