bugfix: fix potential infinite loop in checkcfg (-fanalyzer)

[kmk3]

It looks like it could happen if a line in /etc/firejail/firejail.config
starts with netfilter-default and there is a space or tab right after
that.

$ pacman -Q gcc14 glibc
gcc14 14.3.1+r25+g42e99e057bd7-1
glibc 2.42+r3+gbc13db739377-1
$ ./configure --enable-analyzer CC=gcc-14 >/dev/null &&
  make clean >/dev/null && make >/dev/null
[...]
../../src/firejail/checkcfg.c: In function ‘checkcfg’:
../../src/firejail/checkcfg.c:137:40: warning: infinite loop [CWE-835] [-Wanalyzer-infinite-loop]
  137 |                                 while (*fname == ' ' || *fname == '\t')
      |                                        ^~~~~~
  ‘checkcfg’: events 1-5
    |
    |  137 |                                 while (*fname == ' ' || *fname == '\t')
    |      |                                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                                        |             |
    |      |                                        |             (2) if it ever follows ‘true’ branch, it will always do so...
    |      |                                        (1) infinite loop here
    |      |                                        (5) ...to here
    |  138 |                                         ptr++;
    |      |                                         ~~~~~
    |      |                                            |
    |      |                                            (3) ...to here
    |      |                                            (4) looping back...
    |
[...]

Added on commit 340a6b2 ("added netfilter-default config option in
/etc/firejail/firejail.config", 2016-07-28).