This is due both how read and write operation update the buffer position,
taking in consideration buffer lenght instead of maximum position defined
by the open mode. This patch fixes it and also fixes fseek not returning
EINVAL for invalid whence modes.
Tested on x86_64 and i686.
[BZ #20012]
* libio/fmemopen.c (fmemopen_read): Use buffer maximum position, not
length to calculate the buffer to read.
(fmemopen_write): Set the buffer position based on bytes written.
(fmemopen_seek): Return EINVAL for invalid whence modes.