cgit_print_snapshot_links(): Free prefix variable

cgit_print_snapshot_links(): Free prefix variable

Fixes following memory leak seen with "PATH_INFO=/cgit/commit/":

    ==16894== 12 bytes in 1 blocks are definitely lost in loss record 9 of 92
    ==16894==    at 0x4C2C04B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
    ==16894==    by 0x56F2DF1: strdup (in /usr/lib/libc-2.17.so)
    ==16894==    by 0x46CAC8: xstrdup (wrapper.c:35)
    ==16894==    by 0x414E34: cgit_print_snapshot_links (ui-shared.c:926)
    ==16894==    by 0x40CFA1: cgit_print_commit (ui-commit.c:102)
    ==16894==    by 0x407B06: commit_fn (cmd.c:54)
    ==16894==    by 0x405E16: process_request (cgit.c:574)
    ==16894==    by 0x4074C8: cache_process (cache.c:322)
    ==16894==    by 0x406C4F: main (cgit.c:872)

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>