buf_string_match_head_str: Fix Coverity issue 'Unsigned compared against 0'

buf_string_match_head_str: Fix Coverity issue 'Unsigned compared against 0'

As Coverity says:
An unsigned value can never be negative, so this test will always
evaluate the same way.

Was changed from int to size_t in commit
7fc608da4ec388c9209bd009cd5053ac0ff7df38 which triggered warning,
but the check did not make sense before, either.

Change-Id: I64f094eeb0ca8c3953a94d742adf468faf27dab3
Signed-off-by: Frank Lichtenheld <frank@lichtenheld.com>
Acked-by: Arne Schwabe <arne-openvpn@rfc2549.org>
Message-Id: <20240119120341.22933-1-gert@greenie.muc.de>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg28093.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit bc29bd6a3376158b73d069758122739fbf93c022)