The CI this week reports a new test failure of the tests/basenc/basenc test,
when compiled with sanitizers.

How to reproduce:
1. Build the current coreutils with
   CC="clang -fsanitize=address,undefined,signed-integer-overflow,shift,integer-divide-by-zero -fno-sanitize-recover=undefined"
   configuring it with option --without-libgmp . (There is no issue when
   coreutils uses the real gmp.)
2. 
$ src/basenc --base58 < /dev/null
../lib/mini-gmp.c:4529:9: runtime error: applying non-zero offset 18446744073709551615 to null pointer
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior ../lib/mini-gmp.c:4529:9 
Aborted

gdb stack trace:

#7  0x0000555555696e79 in __ubsan_handle_pointer_overflow_abort ()
    at /home/runner/work/llvm-project/llvm-project/compiler-rt/lib/ubsan/ubsan_handlers.cpp:855
#8  0x00005555556e27a2 in mpz_import (r=0x7bfff5900060, count=0, order=1, size=1, endian=-1, nails=0, src=0x0) at ../lib/mini-gmp.c:4529
#9  0x00005555556a70a7 in base58_encode (data=0x0, data_len=0, out=0x7edff6de0400 '\276' <repeats 200 times>..., outlen=0x7bfff5b000c0)
    at ../src/basenc.c:1178
#10 0x00005555556a2d56 in base58_encode_ctx_finalize (ctx=0x7bfff5b00060, out=0x7bfff5b00040, outlen=0x7bfff5b000c0)
    at ../src/basenc.c:1200
#11 0x00005555556a65d1 in do_encode (in=0x7ffff7e038e0 <_IO_2_1_stdin_>, infile=0x55555570fd60 <str> "-", 
    out=0x7ffff7e045c0 <_IO_2_1_stdout_>, wrap_column=76) at ../src/basenc.c:1431
#12 0x0000555555699973 in main (argc=2, argv=0x7fffffffcfd8) at ../src/basenc.c:1736

Adding a non-zero offset to a NULL pointer is undefined behaviour per
ISO C 23 § 6.5.7.(9) as amended by N3322.

Should mpz_import accept count=0, src=NULL arguments? Hard to say from the gmp
documentation https://gmplib.org/manual/Integer-Import-and-Export .

If yes, then it's a bug in mini-gmp.c around line 4529.
If no, it's a bug in coreutils/src/basenc.c.

If yes, feel free to report that to the GMP people, without CCing me, please.
If no, feel free to apply the attached fix. It passes "make check".

Bruno