Hello,

We are currently working on fuzz testing feature, and we found a heap-use-after-free on `cflow`.

The stack traces are as follow:
```st
==29241==ERROR: AddressSanitizer: heap-use-after-free on address 0x60e000003ef0 at pc 0x55ccc9783d79 bp 0x7fff15477370 sp 0x7fff15477360
READ of size 8 at 0x60e000003ef0 thread T0
    #0 0x55ccc9783d78 in reference .../cflow-1.6/src/parser.c:1298
    #1 0x55ccc9780af5 in expression .../cflow-1.6/src/parser.c:621
    #2 0x55ccc9782336 in func_body .../cflow-1.6/src/parser.c:1051
    #3 0x55ccc9780d6f in parse_function_declaration .../cflow-1.6/src/parser.c:690
    #4 0x55ccc9780975 in parse_declaration .../cflow-1.6/src/parser.c:578
    #5 0x55ccc978071b in yyparse .../cflow-1.6/src/parser.c:528
    #6 0x55ccc977aa1d in main .../cflow-1.6/src/main.c:812
```
The full stack trace is attached.

Step to reproduce

We configured `cflow` using `CFLAGS="-g -O0 -fsanitize=address" ./configure --disable-shared` and built in using `make -j10; make install`, and run it with:

```
.../cflow-1.6/install_asan/bin/cflow -o --omit-arguments x -m --number t s --debug -o -s <attached file> 24 /dev/null -n
```

The input file 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.