schat: Require explicit path to certificate.

Don't allow using files outside of module data directory.
This is the same requirement as for log path in log module.

modules/schat.cpp

@@ -115,12 +115,13 @@ class CSChat : public CModule {
     ~CSChat() override {}
 
     bool OnLoad(const CString& sArgs, CString& sMessage) override {
-        m_sPemFile = sArgs;
-
-        if (m_sPemFile.empty()) {
-            m_sPemFile = CZNC::Get().GetPemLocation();
+        if (sArgs.empty()) {
+            sMessage = "Argument must be path to PEM file";
+            return false;
         }
 
+        m_sPemFile = CDir::CheckPathPrefix(GetSavePath(), sArgs);
+
         if (!CFile::Exists(m_sPemFile)) {
             sMessage = "Unable to load pem file [" + m_sPemFile + "]";
             return false;