[rtmpdump] AMF_GetProp return garbage for non existent properties

[KSV]

Test url : rtmpdump -r "rtmp://flashmedia.vtc.com/vtcStream" -a "vtcStream" -f "WIN 10,3,181,14" -p "http://www.vtc.com/modules/products/moviePlay.php?size=Large&foldername=lightwave10&sku=34214&id=101286&movieCode=0101&movieName=New+in+Lightwave+10&mode=flv" -C S:valid -y "beee88997dfa34a04f1c2d890753c6183cad3e5ce29e3a31cd321ec24984dd05" -o beee88997dfa34a04f1c2d890753c6183cad3e5ce29e3a31cd321ec24984dd05.flv -V


in above test case object decoding fails if named object contains AMF_NULL (0x05) because
it tries to decode all properties as named. attached patch checks for AMF_NULL before trying 

to decode it's name.

KSV wrote:
AMF_GetProp uses wrong index value for accessing the non existent property and returns garbage instead of AMFProp_Invalid. attached patch fixes the problem.