ac_str: Fix some -Wconversion warnings
ac_str: Fix some -Wconversion warnings

If compiling with -Wconversion the following was thrown against ac_net.c

ac_str.c: In function ‘ac_str_split’:
ac_str.c:79:43: warning: conversion to ‘long unsigned int’ from ‘int’ may change the sign of the result [-Wsign-conversion]
   79 |   fields = realloc(fields, sizeof(char *) * i);
      |                                           ^
ac_str.c:84:42: warning: conversion to ‘long unsigned int’ from ‘int’ may change the sign of the result [-Wsign-conversion]
   84 |  fields = realloc(fields, sizeof(char *) * i);
      |                                          ^
ac_str.c: In function ‘ac_str_levenshtein’:
ac_str.c:183:11: warning: conversion from ‘size_t’ {aka ‘long unsigned int’} to ‘int’ may change value [-Wconversion]
  183 |   v0[i] = i;
      |           ^
ac_str.c:188:11: warning: conversion from ‘size_t’ {aka ‘long unsigned int’} to ‘int’ may change value [-Wconversion]
  188 |   v1[0] = i + 1;
      |           ^

They are likely harmless in general, however the first two can be
properly fixed by making i an unsigned int.

The second two we really just silence the warnings...

Signed-off-by: Andrew Clayton <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@digital-domain.net>

ac000/libac