libopkg/opkg_cmd.c: more robust PATH handling

libopkg/opkg_cmd.c: more robust PATH handling

preserve semantics of PATH when PATH is not set in environment

error and undefined behavior reported in
  https://github.com/openwrt/packages/issues/1922
when PATH not set in environment
(lighttpd executes CGI with empty base env, plus standard CGI env vars)

Signed-off-by: Glenn Strauss <[email protected]>
[Jo-Philipp Wich: avoid free() on NULL, use default from cmake cache string]
Signed-off-by: Jo-Philipp Wich <[email protected]>