If the passed t pointer is not a null pointer, always assign the return
value to the object it points to, regardless of whether the return value
is an error.
This is what the GNU C Library does, and this is also the expected
behavior according to the latest draft of the C programming language
standard (C11 ISO/IEC 9899:201x WG14 N1570, dated 2011-04-12):
Signed-off-by: Benoît Thébaudeau <benoit@wsystem.com>