follow_stream: rework "save as" slightly:

follow_stream: rework "save as" slightly:
  - Prevent "dead" save-as window if unable to open file for write
     or if a directory is entered as the filename;
     Work-around is to always kill save-as window whether success
      or an error causing alert-box popup;
  - Fix a memory leak.
  - Define certain functions used only in follow_stream.c as static;
    (Remove declarations for them from follow_stream.h)

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@28574 f5534014-38df-0310-8fa8-9805f1628bb7