ShellPkg/UefiShellCommandLib: suppress incorrect gcc warning

`Dupes` is used only if `Duplicates != NULL` and function is left if
allocation of memory for `Dupes` fails, so it can't be used
uninitialized.

Ref: https://bugzilla.tianocore.org/show_bug.cgi?id=3228
Cc: Ray Ni <ray.ni@intel.com>
Cc: Zhichao Gao <zhichao.gao@intel.com>
Signed-off-by: Sergei Dmitrouk <sergei@posteo.net>
Message-Id: <20210511225616.5942-2-sergei@posteo.net>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>

ShellPkg/Library/UefiShellCommandLib/UefiShellCommandLib.c

@@ -2128,6 +2128,11 @@ ShellSortFileList (
     }
   }
 
+  //
+  // Set Dupes to suppress incorrect compiler/analyzer warnings.
+  //
+  Dupes = NULL;
+
   //
   // If separation of duplicates has been requested, allocate the list for
   // them.