http: reject header contents with nul bytes

They are not allowed by the protocol and allowing them risk that curl
misbehaves somewhere where C functions are used but won't work on the
full contents. Further, they are not supported by hyper and they cause
problems for the new coming headers API work.

Updated test 262 to verify and enabled it for hyper as well

Closes #8601

lib/http.c

@@ -4283,6 +4283,13 @@ CURLcode Curl_http_readwrite_headers(struct Curl_easy *data,
       }
     }
 
+    end_ptr = memchr(headp, 0x00, Curl_dyn_len(&data->state.headerb));
+    if(end_ptr) {
+      /* this is bad, bail out */
+      failf(data, "Nul byte in header");
+      return CURLE_WEIRD_SERVER_REPLY;
+    }
+
     result = Curl_http_header(data, conn, headp);
     if(result)
       return result;