debug/pe: omit panic in (*File).ImportedSymbols on empty optional headers

If a PE file with invalid optional header size (neither
sizeofOptionalHeader32 nor sizeofOptionalHeader64) is passed to NewFile,
the File.OptionalHeader will be nil which leads to a panic in
(*File).ImportedSymbols().

Fixes #30250

Change-Id: Ie97306de4a0e2dcfdc7b1b599891f574aa63adca
Reviewed-on: https://go-review.googlesource.com/c/162858
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

src/debug/pe/file.go

@@ -324,6 +324,10 @@ type ImportDirectory struct {
 // satisfied by other libraries at dynamic load time.
 // It does not return weak symbols.
 func (f *File) ImportedSymbols() ([]string, error) {
+	if f.OptionalHeader == nil {
+		return nil, nil
+	}
+
 	pe64 := f.Machine == IMAGE_FILE_MACHINE_AMD64
 
 	// grab the number of data directory entries

src/debug/pe/file_test.go

@@ -5,6 +5,7 @@
 package pe
 
 import (
+	"bytes"
 	"debug/dwarf"
 	"internal/testenv"
 	"io/ioutil"
@@ -627,3 +628,22 @@ func TestImportTableInUnknownSection(t *testing.T) {
 		t.Fatalf("unable to locate any imported symbols within file %q.", path)
 	}
 }
+
+func TestInvalidFormat(t *testing.T) {
+	crashers := [][]byte{
+		// https://golang.org/issue/30250
+		[]byte("\x00\x00\x00\x0000000\x00\x00\x00\x00\x00\x00\x000000" +
+			"00000000000000000000" +
+			"000000000\x00\x00\x0000000000" +
+			"00000000000000000000" +
+			"0000000000000000"),
+	}
+
+	for _, data := range crashers {
+		f, err := NewFile(bytes.NewReader(data))
+		if err != nil {
+			t.Error(err)
+		}
+		f.ImportedSymbols()
+	}
+}