git-remote SEGV on t5505 test.

Hello,

The gdb backtrace said:
(gdb) bt
#0  0xff03455c in strlen () from /usr/lib/libc.so.1
#1  0xff087058 in _doprnt () from /usr/lib/libc.so.1
#2  0xff088c18 in printf () from /usr/lib/libc.so.1
#3  0x0007df20 in prune (argc=1556136, argv=0x139cd0) at
builtin-remote.c:590

The codes in builtin-remote.c:590,
         printf(" * [%s] %s\n", dry_run ? "would prune" : "prune",
                skip_prefix(refname, "refs/remotes/"));

And the 'skip_prefix()' returns NULL in this case.
(The old skip_prefix() never returns NULL).

It seems there is another place need to be checked:
   builtin-remote.c:464 : can pass NULL to the strdup().

Regards,
namsh