Fix jumps depending on unitialized values

Reported by valgrind

src/eval.c

@@ -10050,7 +10050,7 @@ getwinvar (
   char_u      *varname;
   dictitem_T  *v;
   tabpage_T   *tp = NULL;
-  tabpage_T   *oldtabpage;
+  tabpage_T   *oldtabpage = NULL;
   int done = FALSE;
 
   if (off == 1)

src/os/rstream.c

@@ -47,6 +47,7 @@ RStream * rstream_new(rstream_cb cb,
   rv->stream = NULL;
   rv->fread_idle = NULL;
   rv->free_handle = false;
+  rv->file_type = UV_UNKNOWN_HANDLE;
 
   return rv;
 }