lib/suitar: let WriteHeader write a directory entry

WriteHeader copied h into w.header before calling w.Write to emit the
GNUTYPE_LONGNAME block holding the name. Write only accepts TypeReg and
TypeGNUSparse, so a TypeDir header failed that call and left the Writer
permanently in an error state - every later entry on the same Writer
failed too.

Set w.header after the name block has gone out instead.

lib/suitar/suitar.go

diff --git a/lib/suitar/suitar.go b/lib/suitar/suitar.go
index 9da6a16..3a6212c 100644
--- a/lib/suitar/suitar.go
+++ b/lib/suitar/suitar.go
@@ -322,10 +322,8 @@
 		w.err = errBadHeader
 		return w.err
 	}
-	w.header = *h
-
-	n := len(w.header.Name)
-	copy(w.nameBuf[:], w.header.Name)
+	n := len(h.Name)
+	copy(w.nameBuf[:], h.Name)
 	w.nameBuf[n] = 0
 
 	initBlock(&w.block, typeGNULongName, int64(n+1), Mode644, 0)
@@ -334,6 +332,10 @@
 		return w.err
 	}
 
+	// The w.Write call below writes this entry's name, not its contents, so
+	// w.header must not (yet) say TypeDir or Write would reject it. Set
+	// w.header after that call.
+	w.header = Header{Typeflag: TypeReg}
 	w.remaining = int64(n + 1)
 
 	if _, err := w.Write(w.nameBuf[:n+1]); err != nil {
@@ -344,6 +346,7 @@
 		return w.err
 	}
 
+	w.header = *h
 	initBlock(&w.block, w.header.Typeflag, w.header.Size, w.header.Mode, w.header.ModTime.Unix())
 	if _, err := w.w.Write(w.block[:]); err != nil {
 		w.err = err