mbrtoc32 returns wrong value for empty input

[bhaible]

When given an empty input, the mbrtoc32 function should return (size_t)(-2). This is clear from ISO C 23, section 7.30.1.5: "If s is not a null pointer, the mbrtoc32 function inspects at most n bytes beginning with the byte pointed to by s to determine the number of bytes needed to complete the next multibyte character (including any shift sequences)." If n is 0, the mbrtoc32 function cannot inspect anything, thus no complete multibyte character can be parsed in such a call.

How to reproduce:

$ gcc -Wall -ggdb foo.c
$ ./a.out
ret = 0
Abort

[bhaible]

test case

[waddlesplash]

Fixed in hrev56914.