libversion: Fix unsafe concatenation of $LD_LIBRARY_PATH

Naive concatenation of $LD_LIBRARY_PATH can result in an empty
colon-delimited segment; this tells glibc to load libraries from the
current directory, which is definitely wrong, and may be a security
vulnerability if the current directory is untrusted.  This particular
case probably has no security relevance, but we should avoid this
unsafe pattern anyway in case it gets copied.  See #76804.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>