[TAR] Apply base-256 encoding to negative values as well

Sources/TAR/ByteReader+Tar.swift

@@ -22,9 +22,7 @@ extension ByteReader {
      */
     func tarCString(maxLength: Int) -> String {
         var buffer = self.bytes(count: maxLength)
-        guard !buffer.isEmpty
-            else { return "" }
-        if buffer.last! != 0 {
+        if buffer.last != 0 {
             buffer.append(0)
         }
         return buffer.withUnsafeBufferPointer { String(cString: $0.baseAddress!) }

Sources/TAR/TarEntryInfo.swift

@@ -361,7 +361,7 @@ fileprivate extension Data {
         }
 
         let maxOctalValue = (1 << (maxLength * 3)) - 1
-        guard value > maxOctalValue else {
+        guard value > maxOctalValue || value < 0 else {
             // Normal octal encoding.
             self.append(String(value, radix: 8).data(using: .utf8)!.zeroPad(maxLength))
             return