debug/pe: validate the number of data directory entries

Updated file.go to validate the number of data directory entries
before assuming the import data directory entry exists. This is
not too common, but it _is_ possible so let's be defensive jic.

debug/pe/file.go: validate NumberOfRvaAndSizes includes imports

Author: arizvisa

src/debug/pe/file.go

@@ -261,6 +261,21 @@ type ImportDirectory struct {
 func (f *File) ImportedSymbols() ([]string, error) {
 	pe64 := f.Machine == IMAGE_FILE_MACHINE_AMD64
 
+	// grab the number of data directory entries
+	var ddcount uint32
+	if pe64 {
+		ddcount = f.OptionalHeader.(*OptionalHeader64).NumberOfRvaAndSizes
+	} else {
+		ddcount = f.OptionalHeader.(*OptionalHeader32).NumberOfRvaAndSizes
+	}
+
+	// if the import data directory doesn't actually exist then we can
+	// assume that there's no symbols
+	if ddcount < 2 {
+		return nil, nil
+	}
+
+	// grab the import data directory entry
 	var idd DataDirectory
 	if pe64 {
 		idd = f.OptionalHeader.(*OptionalHeader64).DataDirectory[1]