ID3v2: Bounds check a vector's size

If the provided vector is empty then `data[0]` dereferences a nullptr,
so add a check that this won't be the case before reading the encoding.

taglib/mpeg/id3v2/frames/ownershipframe.cpp

@@ -117,6 +117,11 @@ void OwnershipFrame::parseFields(const ByteVector &data)
 {
   int pos = 0;
 
+  // Need at least 1 byte for the encoding
+  if(data.size() - pos < 1) {
+    return;
+  }
+
   // Get the text encoding
   d->textEncoding = String::Type(data[0]);
   pos += 1;