added more error-checking to sftp file read/write

rsmcore/session.cc

@@ -239,6 +239,7 @@ bool SFTPFile::isOpen() { return (p->file != nullptr); }
 // on error, returns < 0 and ssh and sftp session errors are set
 int64_t SFTPFile::write(QString str)
 {
+  if (nullptr == p->file) return -1;
   std::string stdStr = str.toStdString();
   const char* buf    = stdStr.c_str();
   size_t count       = strlen(buf);
@@ -250,7 +251,7 @@ int64_t SFTPFile::write(QString str)
 // success
 QString SFTPFile::read(size_t nBytes)
 {
-  if (nBytes == 0) return QString();
+  if (nullptr == p->file || nBytes == 0) return QString();
   char* buf = new char[nBytes + 1];
   // initialize buff
   memset(buf, 0, nBytes + 1);