fix interactive Corba crash

- guard SystemImpl__directoryExists for NULL string
- guard CorbaImpl__initialize agains NULL corbaObjectReferenceFilePath

Author: adrpo

Compiler/runtime/corbaimpl.cpp

@@ -226,7 +226,7 @@ Please stop or kill the other OMC process first!\nOpenModelica OMC will now exit
   /* get the temporary directory */
   /* if corbaObjectReferenceFilePath value is set and is a valid directory then use it for dumping the object reference file. */
   const char* tempPath = NULL;
-  if (SystemImpl__directoryExists(corbaObjectReferenceFilePath))
+  if (!corbaObjectReferenceFilePath && SystemImpl__directoryExists(corbaObjectReferenceFilePath))
     tempPath = corbaObjectReferenceFilePath;
   else
     tempPath = SettingsImpl__getTempDirectoryPath();

Compiler/runtime/systemimpl.c

@@ -851,6 +851,8 @@ extern double SystemImpl__time(void)
 
 extern int SystemImpl__directoryExists(const char *str)
 {
+  /* if the string is NULL return 0 */
+  if (!str) return 0;
 #if defined(__MINGW32__) || defined(_MSC_VER)
   WIN32_FIND_DATA FileData;
   HANDLE sh;