rstream: Initialize 'rstream' field in RBuffer class

This field is used in a conditional jump, so this initialization is fixing a
bug.

src/nvim/os/rstream.c

@@ -47,6 +47,7 @@ RBuffer *rbuffer_new(size_t capacity)
   rv->data = xmalloc(capacity);
   rv->capacity = capacity;
   rv->rpos = rv->wpos = 0;
+  rv->rstream = NULL;
   return rv;
 }