terminal: Handle loss of focus in event loop.

While in a terminal and insert mode, if an event caused loss of focus,
nvim would stay in the terminal event loop causing an inconsistent view
of internal state and/or segfault.

Remove the "term" argument from terminal_enter() as it only makes sense
to call it with curbuf->terminal. Terminate the loop when switched to a
different buffer.

fixes #2301

src/nvim/edit.c

@@ -246,7 +246,7 @@ edit (
 )
 {
   if (curbuf->terminal) {
-    terminal_enter(curbuf->terminal, true);
+    terminal_enter(true);
     return false;
   }