Instead of checking Fcall.data==nil, check Fcall.count==0.
The former check always fails after `gcc -O2` optimizations
(gcc version 8.3.0).
Also fix an out-of-bound read detected by valgrind:
```
==31162== Invalid read of size 1
==31162== at 0x11005E: morerules (rules.c:739)
==31162== by 0x110254: writerules (rules.c:775)
==31162== by 0x10D2FE: fsyswrite (fsys.c:848)
==31162== by 0x10C304: fsysproc (fsys.c:248)
==31162== by 0x112E8C: threadstart (thread.c:96)
==31162== by 0x4A682BF: ??? (in /usr/lib/libc-2.29.so)
==31162== Address 0x4ea984a is 0 bytes after a block of size 250 alloc'd
==31162== at 0x483AD7B: realloc (vg_replace_malloc.c:826)
==31162== by 0x1196F3: p9realloc (malloc.c:53)
==31162== by 0x10BDFD: erealloc (plumber.c:124)
==31162== by 0x10FCD9: concat (rules.c:642)
==31162== by 0x10FCD9: concat (rules.c:635)
==31162== by 0x110230: writerules (rules.c:773)
==31162== by 0x10D2FE: fsyswrite (fsys.c:848)
==31162== by 0x10C304: fsysproc (fsys.c:248)
==31162== by 0x112E8C: threadstart (thread.c:96)
==31162== by 0x4A682BF: ??? (in /usr/lib/libc-2.29.so)
```
Fixes#256
