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syntax.c: Fix maybe-uninitialized warning (#7596)

When building in release mode, gcc generated a maybe-initialized
warning in get_syn_options. The warning is both right and wrong;
there is an execution path where the len variable is not
initialized in the code:
...
int len;
...
for (fidx = ARRAY_SIZE(flagtab); --fidx >= 0; ) {
  p = flagtab[fidx].name;
  int i;
  for (i = 0, len = 0; p[i] != NUL; i += 2, ++len)
    if (arg[len] != p[i] && arg[len] != p[i + 1])
      break;
  // <snip>
}
...
  arg = skipwhite(arg + len);
...

The initial for loop will not execute if ARRAY_SIZE(flagtab) == 0,
and thus len will never be initialized. flagtab is a local-static
variable, initialized to a long array of structured data, so
ARRAY_SIZE(flagtab) can't be 0.

However, gcc doesn't recognize ARRAY_SIZE(flagtab) as a constant.
There are any number of reasons this could happen. In any case,
the message can be fixed with a len=0 before the first for loop.

In addition to the above warning, I've labeled flagtab and
first_letters as const. They should never change.

2 of 2 new or added lines in 1 file covered. (100.0%)

87771 of 113450 relevant lines covered (77.37%)

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