utils: fragmented_temporary_buffer: fix view::remove_prefix
utils: fragmented_temporary_buffer: fix view::remove_prefix

This piece of logic was wrong for two unrelated reasons:
1. When fragmented_temporary_buffer::view is constructed from bytes_view,
_current is null. When remove_prefix was used on such view, null pointer
dereference happened.
2. It only worked for the first remove_prefix call. A second call would put a
wrong value in _current_position.

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