rtmpproto: avoid setting auth var value if null

Use default values if parsed variable is found not to
have any value. Avoids crashing at strlen for salt/user
on the auth call afterwards and needless NULL assignments
for the rest (default is already NULL for those).

Should fix Coverity Scan issues #966644 and #966645

Signed-off-by: Reynaldo H. Verdejo Pinochet <r.verdejo@sisa.samsung.com>
