Fix 'error return without exception set'

Based on Marius's #865, with some extra checking and a test.

src/runtime/objmodel.cpp

@@ -2836,9 +2836,10 @@ Box* _callattrEntry(Box* obj, BoxedString* attr, CallattrFlags flags, Box* arg1,
         assert(!(S == CAPI && flags.null_on_nonexistent));
         if (!rewrite_args.out_success) {
             rewriter.reset(NULL);
-        } else if (rtn) {
+        } else if (rtn || S == CAPI) {
             rewriter->commitReturning(rewrite_args.out_rtn);
-        } else if ((S == CAPI && !rtn) || flags.null_on_nonexistent) {
+        } else if (flags.null_on_nonexistent) {
+            assert(!rewrite_args.out_rtn);
             rewriter->commitReturning(rewriter->loadConst(0, rewriter->getReturnDestination()));
         }
     } else {

test/tests/exceptions_regressions.py

+class C(object):
+    def f(self, n):
+        if n == 5000:
+            return
+        else:
+            raise Exception()
+
+def f(c, i):
+    try:
+        c.f(i)
+    except Exception:
+        pass
+
+c = C()
+for i in xrange(10000):
+    f(c, i)