[qtmozembed] Guard against null certificate data. Contributes to JB#4…

…9873

The nsIX509Cert::GetRawDER() call can fail and return a null data
pointer under certain circumstances. This wasn't being guarded against
and so had the potential to crash the browser when opening an HTTPS
page. This change performs checks before attempting to use the returned
value.

src/qmozsecurity.cpp

@@ -288,10 +288,13 @@ void QMozSecurity::importState(const char *aStatus, unsigned int aState)
             emissions << &QMozSecurity::protocolVersionChanged;
         }
 
+        QSslCertificate serverCertificate;
         uint32_t length;
         char *data;
-        aServerCert->GetRawDER(&length, (uint8_t **)&data);
-        QSslCertificate serverCertificate = QSslCertificate(QByteArray(data, length), QSsl::EncodingFormat::Der);
+        nsresult rv = aServerCert->GetRawDER(&length, (uint8_t **)&data);
+        if (rv == NS_OK && data) {
+            serverCertificate = QSslCertificate(QByteArray(data, length), QSsl::EncodingFormat::Der);
+        }
         if (m_serverCertificate != serverCertificate) {
             m_serverCertificate = serverCertificate;
             emissions << &QMozSecurity::serverCertificateChanged;