fix complie error: specified bound depends on the length of the source argument

[drawing]

Fix compilation errors under gcc version 11.3.1 20221121 (Red Hat 11.3.1-4)

/tmp/build/xquic-1.5.2/src/tls/xqc_tls.c: In function 'xqc_tls_set_alpn': /tmp/build/xquic-1.5.2/src/tls/xqc_tls.c:172:5: error: 'strncpy' specified bound depends on the length of the source argument [-Werror=stringop-truncation] 172 | strncpy(&p_alpn[1], alpn, protos_len); | ^~~~~~~~~ /tmp/build/xquic-1.5.2/src/tls/xqc_tls.c:152:23: note: length computed here 152 | size_t alpn_len = strlen(alpn); | ^~~~ cc1: all warnings being treated as errors

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[Kulsk]

LGTM