Use of unitialized value in crypto_pk_num_bits (src/common/crypto.c:971)

Triggered in `51e4748`, compiled with clang 6.0.0-trunk and -fsanitize=memory. `./fuzz-hsdescv2 < test002` ``` ==20245==WARNING: MemorySanitizer: use-of-uninitialized-value #0 0x558a63e2d816 in crypto_pk_num_bits /root/tor/src/common/crypto.c:971:3 #1 0x558a63b8f66a in token_check_object /root/tor/src/or/parsecommon.c:212:23 #2 0x558a63b8f66a in get_next_token /root/tor/src/or/parsecommon.c:397 #3 0x558a63b8d097 in tokenize_string /root/tor/src/or/parsecommon.c:72:11 #4 0x558a63ce97f7 in rend_parse_v2_service_descriptor /root/tor/src/or/routerparse.c:5197:7 #5 0x558a63889d02 in fuzz_main /root/tor/src/test/fuzz/fuzz_hsdescv2.c:40:10 #6 0x558a6388977a in main /root/tor/src/test/fuzz/fuzzing_common.c:179:3 #7 0x7fa41c9773f0 in __libc_start_main /build/glibc-mXZSwJ/glibc-2.24/csu/../csu/libc-start.c:291 #8 0x558a638177d9 in _start (/root/tor/src/test/fuzz/fuzz-hsdescv2+0x717d9) SUMMARY: MemorySanitizer: use-of-uninitialized-value /root/tor/src/common/crypto.c:971:3 in crypto_pk_num_bits ```