stdbuf: avoid even the appearance of a possible use-after-free

stdbuf: avoid even the appearance of a possible use-after-free

There was an execution path by which "libstdbuf" could be used after
being freed, but that would happen only if there were no libstdbuf.so
alongside the stdbuf program and there had been an installation error
leading to absence of the file, PKGLIBDIR/libstdbuf.so.
* src/stdbuf.c (set_LD_PRELOAD): Rearrange loop to make it perfectly
clear that there is no possibility of use-after-free.
Steve Grubb reported this possible use-after-free of "libstdbuf".