bmcweb: Fix issues with /dump endpoint

Technically, if multiple dump files existed, we could get a double end()
called loading each file.  Avoid that by returning.

Tested By:
Downloading a file from dump, using the /dump url.  File downloads 
correctly.

Change-Id: Icd71a2fbbe365557daa1a7e24c5563f3749a748d
Signed-off-by: Ed Tanous <ed.tanous@intel.com>