help: don't cycle forward in the buffers when exiting from help

When nano has multiple files open, closing a help buffer should
not switch to the next buffer in the ring but to the preceding one,
because it was from there that the help screen was invoked.

src/files.c

@@ -611,7 +611,7 @@ void switch_to_prevnext_buffer(bool to_next)
     assert(openfile != NULL);
 
     /* If only one file buffer is open, say so and get out. */
-    if (openfile == openfile->next) {
+    if (openfile == openfile->next && !inhelp) {
 	statusbar(_("No more open file buffers"));
 	return;
     }

src/help.c

@@ -235,6 +235,9 @@ void do_help(void)
 	}
     }
 
+    /* Switch back to the buffer we were invoked from. */
+    switch_to_prev_buffer_void();
+
     /* Restore the settings of all flags. */
     memcpy(flags, stash, sizeof(flags));