MdeModulePkg/UdfDxe: Content check for 'File' in ResolveSymlink()

MdeModulePkg/UdfDxe: Content check for 'File' in ResolveSymlink()

REF:https://bugzilla.tianocore.org/show_bug.cgi?id=1279

The content within 'File' is the output data for ResolveSymlink(). This
commit will add checks to ensure the content in 'File' is valid.
Otherwise, possible null pointer dereference issue will occur during the
subsequent usage of the data returned by ResolveSymlink().

Cc: Ruiyu Ni <ruiyu.ni@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Hao Wu <hao.a.wu@intel.com>
Reviewed-by: Paulo Alcantara <palcantara@suse.de>
Reviewed-by: Star Zeng <star.zeng@intel.com>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>