There is a bug in ext/iconv/iconv.c file on line 234.

When you do not use libiconv and the output (translated) string is longer than the input string, you obtain a wrong length of the output string.

The problem is that there is an output buffer estimate (input buffer length + 32 bytes):

..
out_left = in_len + 32; /* Avoid realloc() most cases */ 
bsz = out_left;
..

but during output buffer realloc there is in_len used instead of bsz:

..
out_left = in_len;
..
out_size += bsz - out_left;
..

The correction should by made on line 234:

out_left = in_len;

should by:

out_left = bsz;