Fix shutdown_callback or returning NULL contract for meta_request

[waahm7]

Issue #, if available:
aws/aws-sdk-java-v2#5277

Description of changes:
There are multiple places where a meta-request can fail after doing the aws_s3_meta_request_init_base like

aws-c-s3/source/s3_auto_ranged_get.c

Line 98 in 9639492

if (aws_s3_parse_request_range_header(

,

aws-c-s3/source/s3_client.c

Line 994 in 9639492

if (!client->s3express_provider) {

,

aws-c-s3/source/s3_client.c

Line 1055 in 9639492

if (endpoint == NULL) {

. In those cases, we were triggering the shutdown callback as well as returning NULL, which is a crime and can lead to double-free errors. This PR moves the shutdown_callback assignment to be the last thing set on the meta_request so that we don’t trigger the shutdown_callback if we fail for any reason after creating the meta_request and before returning it to customers.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[graebm]

looks good overall, but had questions