debug/dwarf: better error handling in SeekPC

The dwarf.Reader "SeekPC" method was not properly handling the case
of a truncated/empty unit (something that has header information
but an empty abbrev table and no DIEs). Add some guards to handle
this case.

Fixes #52045.

Change-Id: I978163eca3b610f7528058693b840931e90d3f63
Reviewed-on: https://go-review.googlesource.com/c/go/+/397054
Reviewed-by: Ian Lance Taylor <[email protected]>
Trust: Than McIntosh <[email protected]>
Run-TryBot: Than McIntosh <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

src/debug/dwarf/entry.go

@@ -974,7 +974,7 @@ 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 {
+		if err != nil || e == nil || e.Tag == 0 {
 			return nil, err
 		}
 		ranges, err := r.d.Ranges(e)

src/debug/dwarf/entry_test.go

@@ -427,3 +427,18 @@ func TestIssue51758(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue52045(t *testing.T) {
+	var abbrev, aranges, frame, line, pubnames, ranges, str []byte
+	info := []byte{0x7, 0x0, 0x0, 0x0, 0x2, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0}
+
+	// A hand-crafted input corresponding to a minimal-size
+	// .debug_info (header only, no DIEs) and an empty abbrev table.
+	data0, _ := New(abbrev, aranges, frame, info, line, pubnames, ranges, str)
+	reader0 := data0.Reader()
+	entry0, _ := reader0.SeekPC(0x0)
+	// main goal is to make sure we can get here without crashing
+	if entry0 != nil {
+		t.Errorf("got non-nil entry0, wanted nil")
+	}
+}