fix(syncthing): avoid writing panic log to nil fd (#10154)

### Purpose

This change fixes a logical bug in the panic log writing where we could
end up writing to a uninitialized file descriptor.

On the very first iteration, `panicFd` is nil. We enter the if `panicFd
== nil { … }` block, check for “panic:” or “fatal error:”, and if
neither matches, we skip instantiating `panicFd` altogether. However,
immediately after, still within `if panicFd == nil { … }`, we call
`panicFd.WriteString("Panic at ...")`. But `panicFd` would in this case
be `nil`, which will cause a run‐time panic.

It's not clear to me why panicFd is only initialized if the lines start
with "panic:" or "fatal error:" so I've left that logic untouched. With
this change we at least avoid the risk of writing to a nil
filedescriptor.
## Authorship

Your name and email will be added automatically to the AUTHORS file
based on the commit metadata.

---------

Co-authored-by: Jakob Borg <jakob@kastelo.net>

cmd/syncthing/monitor.go

@@ -238,10 +238,9 @@ func copyStderr(stderr io.Reader, dst io.Writer) {
 			return
 		}
 
-		if panicFd == nil {
 		dst.Write([]byte(line))
 
-			if strings.HasPrefix(line, "panic:") || strings.HasPrefix(line, "fatal error:") {
+		if panicFd == nil && (strings.HasPrefix(line, "panic:") || strings.HasPrefix(line, "fatal error:")) {
 			panicFd, err = os.Create(locations.GetTimestamped(locations.PanicLog))
 			if err != nil {
 				l.Warnln("Create panic log:", err)
@@ -277,7 +276,6 @@ func copyStderr(stderr io.Reader, dst io.Writer) {
 				panicFd.WriteString(line)
 			}
 			stdoutMut.Unlock()
-			}
 
 			panicFd.WriteString("Panic at " + time.Now().Format(time.RFC3339) + "\n")
 		}