stream: do not initialise file streams with a non-empty vector

Because of the not_inlined trick, a 0 initialised SBO vector is
completely valid to use if required.  Future /vec_ensure/'s will deal
with it appropriately.  So there's no need to initialise the vector
ahead of time like this.

src/stream.c

@@ -83,8 +83,6 @@ stream_err_t stream_init_file(stream_t *stream, const char *name, FILE *pipe)
   stream->name      = name;
   stream->pipe.file = pipe;
 
-  vec_init(&stream->pipe.cache, STREAM_DEFAULT_CHUNK);
-
   return STREAM_ERR_OK;
 }