print_tag_downloads(): Free ref variable
Make sure the ref variable is freed if we build a
"$basename-$version"-style ref.
This fixes following memory leak seen with "PATH_INFO=/cgit/refs/":
==8784== 323 bytes in 29 blocks are definitely lost in loss record 41 of 53
==8784== at 0x4C2C04B: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==8784== by 0x56F2DF1: strdup (in /usr/lib/libc-2.17.so)
==8784== by 0x46CA28: xstrdup (wrapper.c:35)
==8784== by 0x410DA6: print_tag_downloads (ui-refs.c:115)
==8784== by 0x410F02: print_tag (ui-refs.c:141)
==8784== by 0x41128B: cgit_print_tags (ui-refs.c:230)
==8784== by 0x41134D: cgit_print_refs (ui-refs.c:250)
==8784== by 0x407C85: refs_fn (cmd.c:105)
==8784== by 0x405DDF: process_request (cgit.c:566)
==8784== by 0x407490: cache_process (cache.c:322)
==8784== by 0x406C18: main (cgit.c:864)
Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>