Uploads rewriter nil pointer panic

https://sentry.gitlap.com/gitlab/gitlab-workhorse-gitlabcom/issues/17782/

*errors.errorString: runtime error: invalid memory address or nil pointer dereference
  File "/usr/local/go/src/runtime/panic.go", line 423, in gopanic
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/raven.go", line 29, in 1
  File "/usr/local/go/src/runtime/asm_amd64.s", line 437, in call32
  File "/usr/local/go/src/runtime/panic.go", line 423, in gopanic
  File "/usr/local/go/src/runtime/panic.go", line 42, in panicmem
  File "/usr/local/go/src/runtime/sigpanic_unix.go", line 24, in sigpanic
  File "/usr/local/go/src/mime/multipart/multipart.go", line 63, in FormName
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/upload/rewrite.go", line 91, in rewriteFormFilesFromMultipart
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/upload/uploads.go", line 31, in HandleFileUploads
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/upload/accelerate.go", line 28, in func1
  File "/usr/local/go/src/net/http/server.go", line 1422, in ServeHTTP
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/staticpages/error_pages.go", line 74, in func1
  File "/usr/local/go/src/net/http/server.go", line 1422, in ServeHTTP
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/staticpages/deploy_page.go", line 17, in func1
  File "/usr/local/go/src/net/http/server.go", line 1422, in ServeHTTP
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/staticpages/servefile.go", line 57, in func1
  File "/usr/local/go/src/net/http/server.go", line 1422, in ServeHTTP
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/upstream/routes.go", line 172, in func1
  File "/usr/local/go/src/net/http/server.go", line 1422, in ServeHTTP
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/internal/upstream/upstream.go", line 115, in ServeHTTP
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/raven.go", line 33, in func1
  File "/var/cache/omnibus/src/gitlab-workhorse/_build/src/gitlab.com/gitlab-org/gitlab-workhorse/vendor/github.com/getsentry/raven-go/http.go", line 82, in func1
  File "/usr/local/go/src/net/http/server.go", line 1422, in ServeHTTP
  File "/usr/local/go/src/net/http/server.go", line 1862, in ServeHTTP
  File "/usr/local/go/src/net/http/server.go", line 1361, in serve

Observed in production (gitlab.com).

changed title from Nil pointer panic to Uploads rewriter nil pointer panic

This is a pretty rare panic on gitlab.com but it should not happen in the first place.

		p, err := reader.NextPart()
		if err == io.EOF {
			break
		}

		name := p.FormName()

Ha! We do not check for other errors.

assigned to @jacobvosmaer-gitlab

mentioned in merge request !108 (merged)

changed milestone to %8.16

closed via merge request !108 (merged)

mentioned in commit tmaczukin/gitlab-workhorse@aa4addd7