BUILD/MEDIUM: http: remove calls to sprintf()

BUILD/MEDIUM: http: remove calls to sprintf()

OpenBSD complains about this use of sprintf() :

src/proto_http.o(.text+0xb0e6): In function `http_process_request':
src/proto_http.c:4127: warning: sprintf() is often misused, please use snprintf()

Here there's no risk as the strings are way shorter than the buffer size
but let's fix it anyway.