fix: crash in message.combineParts when copying a nil slice

Changelog.md

@@ -23,6 +23,7 @@ Changelog [format](http://keepachangelog.com/en/1.0.0/)
 * GODT-390 Don't logout user if AuthRefresh fails because internet was off
 * GODT-341 Fixed flaky unittest for Store synchronization cooldown
 * Crash when failing to match necessary html element
+* Crash in message.combineParts when copying nil slice
 
 ## [v1.2.7] Donghai-hotfix - beta (2020-05-07)
 

pkg/message/parser.go

@@ -140,6 +140,9 @@ func combineParts(m *pmapi.Message, parts []io.Reader, headers []textproto.MIMEH
 			att := parseAttachment(filename, mediaType, h)
 
 			b := &bytes.Buffer{}
+			if d == nil {
+				continue
+			}
 			if _, err = io.Copy(b, d); err != nil {
 				continue
 			}