CDVD: Check file actually opened before proceeding

If you loaded an invalid file, particularly with compression, the file
open may fail, leading to a division by zero (on block size), or trying
to read from nothing.

pcsx2/CDVD/GzippedFileReader.cpp

@@ -80,7 +80,7 @@ static void WriteIndexToFile(Access* index, const char* filename)
 		return;
 	}
 
-	auto fp = FileSystem::OpenManagedCFile(filename, "rb");
+	auto fp = FileSystem::OpenManagedCFile(filename, "wb");
 	if (!fp)
 		return;
 

pcsx2/CDVD/InputIsoFile.cpp

@@ -221,7 +221,8 @@ bool InputIsoFile::Open(std::string srcfile, bool testOnly)
 		m_reader = new FlatFileReader(EmuConfig.CdvdShareWrite);
 	}
 
-	m_reader->Open(m_filename);
+	if (!m_reader->Open(m_filename))
+		return false;
 
 	// It might actually be a blockdump file.
 	// Check that before continuing with the FlatFileReader.