exceptions: fix nil pointer exception in PyErr_Fetch

Fixes #37

exceptions.go

@@ -61,6 +61,10 @@ func PyErr_Clear() {
 //
 // Note This function is normally only used by code that needs to handle exceptions or by code that needs to save and restore the error indicator temporarily.
 func PyErr_Fetch() (exc, val, tb *PyObject) {
+	exc = &PyObject{}
+	val = &PyObject{}
+	tb = &PyObject{}
+
 	C.PyErr_Fetch(&exc.ptr, &val.ptr, &tb.ptr)
 	return
 }

python_test.go

@@ -108,4 +108,10 @@ cPickle.loads("S'foo'\np1\n.") = "foo"
 	})
 }
 
-// EOF
+func TestErrFetch(t *testing.T) {
+	t.Parallel()
+	testPkg(t, pkg{
+		path: "tests/errfetch",
+		want: []byte("exc=&{<nil>}\nval=&{<nil>}\ntb=&{<nil>}\n"),
+	})
+}

tests/errfetch/main.go

@@ -0,0 +1,19 @@
+package main
+
+import (
+	"fmt"
+
+	"github.com/sbinet/go-python"
+)
+
+func init() {
+	err := python.Initialize()
+	if err != nil {
+		panic(err.Error())
+	}
+}
+
+func main() {
+	exc, val, tb := python.PyErr_Fetch()
+	fmt.Printf("exc=%v\nval=%v\ntb=%v\n", exc, val, tb)
+}