tools/http_load: Possible OOB snprintf

[nnewram]

snprintf returns number possibly written, not actually written.
Second iteration for cookies / extra header, this can cause oob if sufficiently long.

trafficserver/tools/http_load/http_load.c

Line 654 in 8671ddb

hdr_bytes += snprintf(&hdr_buf[hdr_bytes], sizeof(hdr_buf) - hdr_bytes, "Cookie: %s\r\n", cookie);

input supplied as cli argv so probably low-priority but thought i'd file an issue anyway (notice that this occurs on multiple places in this function).