debug/dwarf: return ErrUnknownPC rather than nil on unknown PC

Currently, on e == nil or e.Tag == 0, SeekPC returns with a nil error.
Instead, indicate that the PC is unknown.

Change-Id: I9594296034e2df872e399bd800b00cb565c413c9
Reviewed-on: https://go-review.googlesource.com/c/go/+/473695
Reviewed-by: Carlos Amedee <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>

Author: 4a6f656c

src/debug/dwarf/entry.go

@@ -975,9 +975,12 @@ func (r *Reader) SeekPC(pc uint64) (*Entry, error) {
 		u := &r.d.unit[unit]
 		r.b = makeBuf(r.d, u, "info", u.off, u.data)
 		e, err := r.Next()
-		if err != nil || e == nil || e.Tag == 0 {
+		if err != nil {
 			return nil, err
 		}
+		if e == nil || e.Tag == 0 {
+			return nil, ErrUnknownPC
+		}
 		ranges, err := r.d.Ranges(e)
 		if err != nil {
 			return nil, err