hexdump: fix format and add test

[alfredh]

hexdump is crashing with formatting errors:

Alfreds-MBP-M1:re alfredh$ ./test/retest -v -r test_fmt_hexdump
single testcase: test_fmt_hexdump
retest: libre version 3.4.0 (arm64/Darwin)
using async polling method 'kqueue'
using datapath './test/data'
regular tests:       print: Format: "%08x<-- SIZE ERROR
Assertion failed: (0 && "RE_VA_ARG: arg is not compatible"), function vhprintf, file print.c, line 503.
Error: Signal (6) 0   retest                              0x0000000100b81fc0 btrace + 52
1   retest                              0x0000000100b81ed0 signal_handler + 60
2   libsystem_platform.dylib            0x00000001985ff4a4 _sigtramp + 56
3   libsystem_pthread.dylib             0x00000001985e7ee0 pthread_kill + 288
4   libsystem_c.dylib                   0x0000000198522340 abort + 168
5   libsystem_c.dylib                   0x0000000198521754 err + 0
6   retest                              0x0000000100b68330 vhprintf + 7780
7   retest                              0x0000000100b687d8 re_vfprintf_s + 92
8   retest                              0x0000000100b68d88 _re_fprintf_s + 48
9   retest                              0x0000000100b64a84 hexdump + 128
10  retest                              0x0000000100afe900 test_fmt_hexdump + 72
11  retest                              0x0000000100b40390 test_unit + 200
12  retest                              0x0000000100b4026c test_reg + 96
13  retest                              0x0000000100b145e8 main + 1036
14  dyld                                0x0000000100fe508c start + 520

Abort trap: 6