Fixed strncpy buffer overflow

The compiler was throwing a warning about a buffer overflow.

-Ryan

```
09:59 $ make
gcc -o rtl_acars_ng rtl_acars_ng.c `pkg-config --cflags --libs librtlsdr libusb` -lpthread -lm -O2
In file included from /usr/include/string.h:638:0,
                 from rtl_acars_ng.c:26:
In function ‘strncpy’,
    inlined from ‘print_mesg’ at rtl_acars_ng.c:1272:18:
/usr/include/i386-linux-gnu/bits/string3.h:120:3: warning: call to __builtin___strncpy_chk will always overflow destination buffer [enabled by default]
   return __builtin___strncpy_chk (__dest, __src, __len, __bos (__dest));
   ^
```