Ext4Pkg: Fix double-free in Ext4ReadSymlink

The SymlinkTmp was deallocated unconditionally, so we shouldn't free it
again on EFI_ERROR

Cc: Marvin Häuser <mhaeuser@posteo.de>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Vitaly Cheptsov <vit9696@protonmail.com>
Fixes: e81432fb ("Ext4Pkg: Add symbolic links support")
Signed-off-by: Savva Mitrofanov <savvamtr@gmail.com>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>
Reviewed-by: Pedro Falcato <pedro.falcato@gmail.com>