Backslash in env causes pipeline upload to freeze

[toolmantim]

The following YML seems to cause 3.0 beta 34+ agents to enter an infinite loop during pipeline upload (with high CPU and unbounded memory growth):

steps:
- env:
    BOOM: \ 

To test, save the above to /tmp/pipeline.yml and run the below command (job and agent token don't need to be valid):

$ docker run -it --rm -e BUILDKITE_AGENT_DEBUG=true -e BUILDKITE_AGENT_DEBUG_HTTP=true -v /tmp/pipeline.yml:/tmp/pipeline.yml buildkite/agent:3.0-beta.34 pipeline upload --job 111 --agent-access-token 111 /tmp/pipeline.yml
2017-11-13 12:04:04 DEBUG  Debug mode enabled
2017-11-13 12:04:04 INFO   Reading pipeline config from "/tmp/pipeline.yml"
2017-11-13 12:04:04 DEBUG  Parsing pipeline configuration as YAML

Beta 33 doesn't show this behaviour:

$ docker run -it --rm -e BUILDKITE_AGENT_DEBUG=true -e BUILDKITE_AGENT_DEBUG_HTTP=true -v /tmp/pipeline.yml:/tmp/pipeline.yml buildkite/agent:3.0-beta.33 pipeline upload --job 111 --agent-access-token 111 /tmp/pipeline.yml
2017-11-13 12:04:15 DEBUG  Debug mode enabled
2017-11-13 12:04:15 INFO   Reading pipeline config from "/tmp/pipeline.yml"
2017-11-13 12:04:15 DEBUG  Parsing pipeline configuration as YAML
2017-11-13 12:04:15 DEBUG  ERR: %!s(<nil>)
POST /v3/jobs/abc123/pipelines HTTP/1.1
Host: agent.buildkite.com
User-Agent: buildkite-agent/3.0-beta.33.1802 (linux; amd64)
Content-Length: 93
Content-Type: application/json
Accept-Encoding: gzip