Ext4Pkg: Fix global buffer overflow in Ext4ReadDir

Directory entry structure can contain name_len bigger than size of "."
or "..", that's why CompareMem in such cases leads to global buffer
overflow. So there are two problems. The first is that statement doesn't
check cases when name_len != 0 but > 2 and the second is that we passing
big Length to CompareMem routine.
The correct way here is to check that name_len <= 2 and check for
null-terminator presence

Signed-off-by: Savva Mitrofanov <savvamtr@gmail.com>

Ext4Pkg/Ext4Dxe/Directory.c

@@ -491,11 +491,9 @@ Ext4ReadDir (
 
     // Entry.name_len may be 0 if it's a nameless entry, like an unused entry
     // or a checksum at the end of the directory block.
-    // memcmp (and CompareMem) return 0 when the passed length is 0.
-
-    IsDotOrDotDot = Entry.name_len != 0 &&
-                    (CompareMem (Entry.name, ".", Entry.name_len) == 0 ||
-                     CompareMem (Entry.name, "..", Entry.name_len) == 0);
+    IsDotOrDotDot = Entry.name_len <= 2 &&
+                    ((Entry.name[0] == '.') &&
+                     (Entry.name[1] == '.' || Entry.name[1] == '\0'));
 
     // When inode = 0, it's unused.
     ShouldSkip = Entry.inode == 0 || IsDotOrDotDot;