Hello,

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

The stack traces are as follow:
```st
==3576==ERROR: AddressSanitizer: heap-use-after-free on address 0x60e0000031d0 at pc 0x55f59aaf6c6e bp 0x7ffd06c1c250 sp 0x7ffd06c1c240
READ of size 8 at 0x60e0000031d0 thread T0
    #0 0x55f59aaf6c6d in call .../cflow-1.6/src/parser.c:1284
    #1 0x55f59aaf3adb in expression .../cflow-1.6/src/parser.c:618
    #2 0x55f59aaf5336 in func_body .../cflow-1.6/src/parser.c:1051
    #3 0x55f59aaf430d in parse_variable_declaration .../cflow-1.6/src/parser.c:781
    #4 0x55f59aaf53f0 in func_body .../cflow-1.6/src/parser.c:1061
    #5 0x55f59aaf430d in parse_variable_declaration .../cflow-1.6/src/parser.c:781
    #6 0x55f59aaf53f0 in func_body .../cflow-1.6/src/parser.c:1061
    #7 0x55f59aaf430d in parse_variable_declaration .../cflow-1.6/src/parser.c:781
    #8 0x55f59aaf53f0 in func_body .../cflow-1.6/src/parser.c:1061
    #9 0x55f59aaf430d in parse_variable_declaration .../cflow-1.6/src/parser.c:781
    #10 0x55f59aaf5398 in func_body .../cflow-1.6/src/parser.c:1056
```
The full stack trace is attached.

*Step to reproduce*

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

```
./cflow <attached file>
```

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.