update avatar email address cause panic

[fangdingjun]

• Gitea version (or commit ref): d572336
• Git version:
• Operating system: debian 9.0
• Database (use [x]):
  • [ *] PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
  • Not relevant
• Log gist:
  [Macaron] 2018-08-01 11:33:08: Started POST /user/settings/avatar for 183.17.126.148 [Macaron] PANIC: runtime error: invalid memory address or nil pointer dereference /home/dingjun/source/golang/src/runtime/panic.go:513 (0x4347c8) /home/dingjun/source/golang/src/runtime/panic.go:82 (0x43394d) /home/dingjun/source/golang/src/runtime/signal_unix.go:390 (0x44a751) /home/dingjun/go/src/code.gitea.io/gitea/routers/user/setting/profile.go:122 (0x11159f6) /home/dingjun/go/src/code.gitea.io/gitea/routers/user/setting/profile.go:158 (0x111610f) /home/dingjun/source/golang/src/runtime/asm_amd64.s:523 (0x462fca) /home/dingjun/source/golang/src/reflect/value.go:447 (0x4ca998) /home/dingjun/source/golang/src/reflect/value.go:308 (0x4ca433) /home/dingjun/go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:177 (0x7edd63) /home/dingjun/go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/inject/inject.go:137 (0x7ed6c9) /home/dingjun/go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:121 (0x81b6f0) /home/dingjun/go/src/code.gitea.io/gitea/vendor/gopkg.in/macaron.v1/context.go:112 (0x81b60e) /home/dingjun/go/src/code.gitea.io/gitea/vendor/github.com/go-macaron/session/session.go:186 (0x86f1be)

Description

change avatar email address, click "Update Avatar", got a http error 500 page

Screenshots

[fangdingjun]

pr #4581 fix this issue