Fix #42 panic occurs if zero stack frames (#43)

sentry.go

@@ -202,7 +202,9 @@ func (hook *SentryHook) Fire(entry *logrus.Entry) error {
 			packet.Culprit = err.Error()
 		} else {
 			currentStacktrace := raven.NewStacktrace(stConfig.Skip, stConfig.Context, stConfig.InAppPrefixes)
-			packet.Interfaces = append(packet.Interfaces, currentStacktrace)
+			if currentStacktrace != nil {
+				packet.Interfaces = append(packet.Interfaces, currentStacktrace)
+			}
 		}
 	} else {
 		// set the culprit even when the stack trace is disabled, as long as we have an error

sentry_test.go

@@ -325,6 +325,16 @@ func TestSentryStacktrace(t *testing.T) {
 		if !strings.HasSuffix(frames[0].Filename, expectedPkgErrorsStackTraceFilename) {
 			t.Error("Stacktrace should be taken from err if it implements the pkgErrorStackTracer interface")
 		}
+
+		// zero stack frames
+		defer func() {
+			if err := recover(); err != nil {
+				t.Error("Zero stack frames should not cause panic")
+			}
+		}()
+		hook.StacktraceConfiguration.Skip = 1000
+		logger.Error(message)
+		<-pch // check panic
 	})
 }