api helpers: Save/restore more values in try_enter/try_leave

This fixes memory leak reported by ASAN. This also somehow fixes test40, though 
I have no idea why except that that test yields memory leak report.

src/nvim/api/private/helpers.c

@@ -49,13 +49,17 @@ void try_enter(TryState *const tstate)
     .trylevel = trylevel,
     .got_int = got_int,
     .did_throw = did_throw,
+    .need_rethrow = need_rethrow,
+    .current_exception = current_exception,
     .msg_list = (const struct msglist *const *)msg_list,
     .private_msg_list = NULL,
   };
   trylevel = 1;
   got_int = false;
   did_throw = false;
+  need_rethrow = false;
   msg_list = &tstate->private_msg_list;
+  current_exception = NULL;
 }
 
 /// End try block, set the error message if any and restore previous state
@@ -72,14 +76,17 @@ bool try_leave(const TryState *const tstate, Error *const err)
 {
   const bool ret = !try_end(err);
   assert(trylevel == 0);
+  assert(!need_rethrow);
   assert(!got_int);
   assert(!did_throw);
   assert(msg_list == &tstate->private_msg_list);
   assert(*msg_list == NULL);
+  assert(current_exception == NULL);
   trylevel = tstate->trylevel;
   got_int = tstate->got_int;
   did_throw = tstate->did_throw;
   msg_list = (struct msglist **)tstate->msg_list;
+  current_exception = tstate->current_exception;
   return ret;
 }
 
@@ -96,6 +103,8 @@ void try_start(void)
 /// @return true if an error occurred
 bool try_end(Error *err)
 {
+  // Note: all globals manipulated here should be saved/restored in
+  // try_enter/try_leave.
   --trylevel;
 
   // Without this it stops processing all subsequent VimL commands and

src/nvim/api/private/helpers.h

@@ -6,6 +6,7 @@
 #include "nvim/api/private/defs.h"
 #include "nvim/vim.h"
 #include "nvim/memory.h"
+#include "nvim/ex_eval.h"
 #include "nvim/lib/kvec.h"
 
 #define OBJECT_OBJ(o) o
@@ -90,6 +91,8 @@ typedef struct {
   int trylevel;
   int got_int;
   int did_throw;
+  int need_rethrow;
+  except_T *current_exception;
   struct msglist *private_msg_list;
   const struct msglist *const *msg_list;
 } TryState;