http2: verify success of strchr() in http2_send()

Detected by Coverity.

Error: NULL_RETURNS:
lib/http2.c:1301: returned_null: "strchr" returns null (checked 103 out of 109 times).
lib/http2.c:1301: var_assigned: Assigning: "hdbuf" = null return value from "strchr".
lib/http2.c:1302: dereference: Incrementing a pointer which might be null: "hdbuf".
1300|
1301|     hdbuf = strchr(hdbuf, 0x0a);
1302|->   ++hdbuf;
1303|
1304|     authority_idx = 0;