deinitFrame: don't modify CAPI exception state

If a CAPI exception was set and we called deinitFrame in some rare cases calling the finalizers cleared the CAPI exception.
This would cause us to return NULL even though no exception is set #1273 (for LLVM functions compiled in capi mode)
Unfortunately I could not come up with a testcase which triggers it but I verified that it fixes the problem scipy triggered.

src/runtime/frame.cpp

@@ -253,16 +253,24 @@ void FrameInfo::disableDeinit(FrameInfo* replacement_frame) {
     assert(isDisabledFrame());
 }
 
-extern "C" void deinitFrameMaybe(FrameInfo* frame_info) {
+extern "C" void deinitFrameMaybe(FrameInfo* frame_info) noexcept {
     // Note: this has to match FrameInfo::disableDeinit
     if (!frame_info->isDisabledFrame())
         deinitFrame(frame_info);
 }
 
-extern "C" void deinitFrame(FrameInfo* frame_info) {
+extern "C" void deinitFrame(FrameInfo* frame_info) noexcept {
     // This can fire if we have a call to deinitFrame() that should be to deinitFrameMaybe() instead
     assert(!frame_info->isDisabledFrame());
 
+    Box* err_type, *err_value, *err_tb;
+    bool restore_exc = cur_thread_state.curexc_type != NULL;
+    if (restore_exc) {
+        // preserve the existing exception
+        PyErr_Fetch(&err_type, &err_value, &err_tb);
+        PyErr_Clear();
+    }
+
     assert(cur_thread_state.frame_info == frame_info);
     cur_thread_state.frame_info = frame_info->back;
     BoxedFrame* frame = frame_info->frame_obj;
@@ -288,6 +296,10 @@ extern "C" void deinitFrame(FrameInfo* frame_info) {
     }
 
     Py_CLEAR(frame_info->globals);
+
+    assert(!PyErr_Occurred());
+    if (restore_exc)
+        PyErr_Restore(err_type, err_value, err_tb);
 }
 
 int frameinfo_traverse(FrameInfo* frame_info, visitproc visit, void* arg) noexcept {

src/runtime/types.h

@@ -1307,8 +1307,8 @@ FunctionMetadata* metadataFromCode(Box* code);
 BORROWED(Box*) getFrame(FrameInfo* frame_info);
 BORROWED(Box*) getFrame(int depth);
 void frameInvalidateBack(BoxedFrame* frame);
-extern "C" void deinitFrame(FrameInfo* frame_info);
-extern "C" void deinitFrameMaybe(FrameInfo* frame_info);
+extern "C" void deinitFrame(FrameInfo* frame_info) noexcept;
+extern "C" void deinitFrameMaybe(FrameInfo* frame_info) noexcept;
 int frameinfo_traverse(FrameInfo* frame_info, visitproc visit, void* arg) noexcept;
 extern "C" void initFrame(FrameInfo* frame_info);
 extern "C" void setFrameExcInfo(FrameInfo* frame_info, STOLEN(Box*) type, STOLEN(Box*) value, STOLEN(Box*) tb);