fix error handling in read-{request,response}-body/latin-1

fix error handling in read-{request,response}-body/latin-1

* module/web/request.scm (read-request-body/latin-1):
* module/web/response.scm (read-response-body/latin-1): Detect short
  reads instead of returning a full buffer with the last bits zeroed
  out. (Before the make-string commit, they contained uninitialized
  memory, which was a fairly serious error.)