fix a bug with secondary nerd trees and NERDTreeQuitOnOpen

Author: Martin Grenfell

doc/NERD_tree.txt

@@ -997,6 +997,10 @@ The latest dev versions are on github
 ==============================================================================
 6. Changelog                                               *NERDTreeChangelog*
 
+Next release:
+    - fix a bug where secondary nerd trees (netrw hijacked trees) and
+      NERDTreeQuitOnOpen didnt play nicely, thanks to Curtis Harvey.
+
 3.1.1
     - fix a bug where a non-listed no-name buffer was getting created every
       time the tree windows was created, thanks to Derek Wyatt and owen1
@@ -1087,6 +1091,7 @@ just downloaded pr0n instead.
     Frederic Chanal (nach)
     Alf Mikula
     Lucas S. Buchala
+    Curtis Harvey
 
 ==============================================================================
 8. License                                                   *NERDTreeLicense*

plugin/NERD_tree.vim

@@ -2344,7 +2344,7 @@ endfunction
 "FUNCTION: s:closeTreeIfQuitOnOpen() {{{2
 "Closes the NERD tree window if the close on open option is set
 function! s:closeTreeIfQuitOnOpen()
-    if g:NERDTreeQuitOnOpen
+    if g:NERDTreeQuitOnOpen && s:isTreeOpen()
         call s:closeTree()
     endif
 endfunction