BoxedCApiFunction::tppCall: always check return value before calling checkAndThrowCAPIException()

this gets triggered by M2Crypto which sets an error but returns None.
previously we throw the error in the non rewriter path and ignored it in the rewriter.
this makes it now identical and mote like cpython does it (only looks at the return value)

src/runtime/capi.cpp

@@ -1872,7 +1872,8 @@ Box* BoxedCApiFunction::tppCall(Box* _self, CallRewriteArgs* rewrite_args, ArgPa
         rewrite_args->out_success = true;
     }
 
-    checkAndThrowCAPIException();
+    if (!rtn)
+        throwCAPIException();
     assert(rtn && "should have set + thrown an exception!");
     return rtn;
 }

src/runtime/exceptions.cpp

@@ -202,7 +202,8 @@ extern "C" void raise3(STOLEN(Box*) arg0, STOLEN(Box*) arg1, STOLEN(Box*) arg2)
     if (reraise)
         startReraise();
 
-    assert(!PyErr_Occurred());
+    if (PyErr_Occurred())
+        PyErr_Clear();
     throw exc_info;
 }