Tighten pod archive format check

Fix #1030

dist/res/config/entry_types/types_archives.txt

@@ -201,7 +201,7 @@ entry_types
 		export_ext = "pod";
 		icon = "e_archive";
 		category = "Archives";
-		reliability = 150;
+		reliability = 100;
 	}
 
 	chasm_bin

src/Archive/Formats/PodArchive.cpp

@@ -292,21 +292,25 @@ bool PodArchive::isPodArchive(MemChunk& mc)
 	mc.seek(0, 0);
 	uint32_t num_files;
 	mc.read(&num_files, 4);
+	if (num_files == 0)
+		return false; // 0 files, unlikely to be a valid archive
 
 	// Read id
 	char id[80];
 	mc.read(id, 80);
 
 	// Check size for directory
-	if (mc.getSize() < 84 + (num_files * 40))
+	auto dir_end = 84 + (num_files * 40);
+	if (mc.getSize() < dir_end)
 		return false;
 
 	// Read directory and check offsets
 	FileEntry entry;
 	for (unsigned a = 0; a < num_files; a++)
 	{
 		mc.read(&entry, 40);
-		if (entry.offset + entry.size > mc.getSize())
+		auto end = entry.offset + entry.size;
+		if (end > mc.getSize() || end < dir_end)
 			return false;
 	}
 	return true;
@@ -335,13 +339,16 @@ bool PodArchive::isPodArchive(string filename)
 	file.Seek(0);
 	uint32_t num_files;
 	file.Read(&num_files, 4);
+	if (num_files == 0)
+		return false; // 0 files, unlikely to be a valid archive
 
 	// Read id
 	char id[80];
 	file.Read(id, 80);
 
 	// Check size for directory
-	if (file_size < 84 + (num_files * 40))
+	auto dir_end = 84 + (num_files * 40);
+	if (file_size < dir_end)
 	{
 		file.Close();
 		return false;
@@ -352,7 +359,8 @@ bool PodArchive::isPodArchive(string filename)
 	for (unsigned a = 0; a < num_files; a++)
 	{
 		file.Read(&entry, 40);
-		if (entry.offset + entry.size > file_size)
+		auto end = entry.offset + entry.size;
+		if (end > file_size || end < dir_end)
 			return false;
 	}
 	return true;