Do not append CWD to (empty) LD_LIBRARY_PATH

[resilar]

If LD_LIBRARY_PATH is unset, nvidia-xinitrc exports LD_LIBRARY_PATH with
a trailing colon (':') which is interpreted as an empty path, i.e., the
current working directory (CWD). This creates a security vulnerability
known as "Uncontrolled Search Path Element" (CWE-427). The commit avoids
appending a trailing colon if LD_LIBRARY_PATH is empty.

[resilar]

For more information, see the links below.

• Stack Overflow: how to smart append LD_LIBRARY_PATH in shell when nounset
• Stack Overflow: What corner cases must we consider when parsing $PATH on Linux?
• How Torch broke ls and made me vulnerable (or the hidden dangers of LD_LIBRARY_PATH)