Fix some potential crashes

The email crash report feature is pretty helpful sometimes

src/MainEditor/UI/ArchivePanel.cpp

@@ -1535,7 +1535,7 @@ bool ArchivePanel::importEntry()
 						ok = false;
 				}
 				// Warn if the offsets couldn't be written
-				if (ok && !si.getFormat()->writeOffset(si, selection[a], offset))
+				if (ok && si.getFormat() && !si.getFormat()->writeOffset(si, selection[a], offset))
 					wxLogMessage("Old offset information [%d, %d] couldn't be "
 					             "preserved in the new image format for image %s.",
 					             offset.x, offset.y, selection[a]->getName());

src/Utility/PropertyList/PropertyList.cpp

@@ -59,7 +59,7 @@ PropertyList::~PropertyList()
 bool PropertyList::propertyExists(string key)
 {
 	// Try to find specified key
-	if (properties.find(key) == properties.end())
+	if (properties.empty() || properties.find(key) == properties.end())
 		return false;
 	else
 		return true;

src/Utility/Tokenizer.cpp

@@ -279,6 +279,9 @@ bool Tokenizer::incrementCurrent()
  *******************************************************************/
 void Tokenizer::skipLineComment()
 {
+	if (atEnd())
+		return;
+
 	// Increment position until a newline or end is found
 	while (current[0] != '\n' && current[0] != 13)
 	{
@@ -295,6 +298,9 @@ void Tokenizer::skipLineComment()
  *******************************************************************/
 void Tokenizer::skipMultilineComment()
 {
+	if (atEnd())
+		return;
+
 	// Increment position until '*/' or end is found
 	while (!(current[0] == '*' && current[1] == '/'))
 	{