Sharing: Upload panics if file has been deleted

[lastzero]

A worker panic may occur if you have selected files for upload that no longer exist, for example because they have been deleted:

2023-05-04 11:00:22 WARN share: share: runtime error: invalid memory address or
nil pointer dereference (worker panic) stack: goroutine 14703 [running]:
runtime/debug.Stack() /usr/local/go/src/runtime/debug/stack.go:24 +0x65
github.com/photoprism/photoprism/internal/workers.(*Share).Start.func1()
/go/src/github.com/photoprism/photoprism/internal/workers/share.go:42 +0x45
panic({0x10440a0, 0x28354e0}) /usr/local/go/src/runtime/panic.go:890 +0x263
github.com/photoprism/photoprism/internal/workers.(*Share).Start(0xc00052ffc8)
/go/src/github.com/photoprism/photoprism/internal/workers/share.go:110 +0xac5
github.com/photoprism/photoprism/internal/workers.RunShare.func1()
/go/src/github.com/photoprism/photoprism/internal/workers/workers.go:96 +0x35
created by github.com/photoprism/photoprism/internal/workers.RunShare
/go/src/github.com/photoprism/photoprism/internal/workers/workers.go:94 +0x6a

Instead, the worker must verify that a record exists before proceeding with the upload.