abort() when vterm_new() is initialized with cols=0 and rows=0

[metan]

The vterm_new() function happily creates a vterm instance with cols=0 and rows=0 and the library aborts later on in resize_buffer() when vterm_set_size() is called as it fails to determine new cursor position. I suppose that the case when old_rows == 0 and old_cols == 0 is not handled in the resize_buffer() code.