MdeModulePkg FvSimpleFileSystemDxe: Fix memory leak in Read function

FvSimpleFileSystem on read always allocates a FileBuffer, and never frees
it. This causes memory leaks. It is especially bad for reading scripts
line-by-line. In some cases memory leak can exceed 1GB.

Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Vladimir Olovyannikiov <vladimir.olovyannikov@broadcom.com>
Reviewed-by: Star Zeng <star.zeng@intel.com>