Hello,

We are currently working on fuzz testing feature, and we found a buffer overflow on `cflow`.

The stack traces are as follow:
==628==ERROR: AddressSanitizer: global-buffer-overflow on address 0x556fe0808482 at pc 0x7f3e9c88abb5 bp 0x7ffcd85b89f0 sp 0x7ffcd85b8198
READ of size 4 at 0x556fe0808482 thread T0
    #0 0x7f3e9c88abb4  (/usr/lib/x86_64-linux-gnu/libasan.so.4+0xafbb4)
    #1 0x556fe07ca85d in find_option_type .../cflow-1.6/src/main.c:238
    #2 0x556fe07cb9ac in set_level_indent .../cflow-1.6/src/main.c:478
    #3 0x556fe07cbc56 in parse_opt .../cflow-1.6/src/main.c:545
    #4 0x556fe07e97cb in group_parse .../cflow-1.6/gnu/argp-parse.c:236
    #5 0x556fe07ec73d in parser_parse_opt .../cflow-1.6/gnu/argp-parse.c:750
    #6 0x556fe07ed222 in parser_parse_next .../cflow-1.6/gnu/argp-parse.c:862
    #7 0x556fe07ed744 in argp_parse .../cflow-1.6/gnu/argp-parse.c:930
    #8 0x556fe07cc7fc in main .../cflow-1.6/src/main.c:788
    #9 0x7f3e9c40bbf6 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21bf6)
    #10 0x556fe07bd659 in _start (.../cflow-1.6/install_asan/bin/cflow+0xf659)

The full stack trace and output are attached.

*Step to reproduce*

We configured `cflow` using

$ CFLAGS="-g -O0 -fsanitize=address" ./configure --disable-shared
, and built it using

$ make -j10; make install

, and run it with:

$ ./cflow --number --tree -o /dev/null <attached file>

The input fila is attached

*Environment*
- OS: Ubuntu 18.04.5 LTS
- GCC version: gcc 7.5.0
- cflow version: cflow 1.6 (https://ftp.gnu.org/gnu/cflow/cflow-1.6.tar.xz)

Thank you.