Ext4Pkg: Check VolumeName allocation correctness in Ext4GetVolumeName

Missing check in some cases leads to failed StrCpyS call in
Ext4GetVolumeLabelInfo. Also correct condition that checks Inode pointer
for being NULL in Ext4AllocateInode

Cc: Marvin Häuser <mhaeuser@posteo.de>
Cc: Pedro Falcato <pedro.falcato@gmail.com>
Cc: Vitaly Cheptsov <vit9696@protonmail.com>
Fixes: cfbbae59 ("Ext4Pkg: Add handling of EFI_FILE_SYSTEM_VOLUME_LABEL GetInfo().")
Signed-off-by: Savva Mitrofanov <savvamtr@gmail.com>
Reviewed-by: Pedro Falcato <pedro.falcato@gmail.com>
Reviewed-by: Marvin Häuser <mhaeuser@posteo.de>