add error tolerance for invalid extents header signature

[joachimmetz]

Seen in a dirty file system

libfsext_extents_header_read_data: extents header data:
00000000: 0a f3 01 00 04 00 01 00  00 00 00 00               ........ ....

libfsext_extents_header_read_data: signature                            : 0xf30a
libfsext_extents_header_read_data: number of extents                    : 1
libfsext_extents_header_read_data: maximum number of extents            : 4
libfsext_extents_header_read_data: depth                                : 1
libfsext_extents_header_read_data: generation                           : 0

libfsext_extent_index_read_data: extent index data:
00000000: 00 00 00 00 27 92 78 00  00 00 7b 00               ....'.x. ..{.

libfsext_extent_index_read_data: logical block number                   : 0
libfsext_extent_index_read_data: physical block number (lower)          : 7901735
libfsext_extent_index_read_data: physical block number (upper)          : 0
libfsext_extent_index_read_data: unknown1                               : 0x007b
libfsext_extent_index_read_data: physical block number                  : 7901735

libfsext_extents_read_file_io_handle: reading extents data of size: 4096 at offset: 32365506560 (0x789227000)
libfsext_extents_header_read_data: extents header data:
00000000: 00 00 00 00 00 00 00 00  00 00 00 00               ........ ....

libfsext_extents_header_read_data: invalid signature.