[mtp] fix possible null dereference in MTPResponder::getObjectPropListReq()

One shouldn't check wheter QMap contains a key using operator[] since it
inserts a default value for the key if it isn't already there.

In this particular case the excess property codes that
PropertyPod::getObjectPropListReq() could insert into some of the
m_objPropMap* QMaps resulted in excess values to be put into the static
QVectors of PropertyPod::getObjectPropsSupportedByType(), if they were
initialized afterwards.

Consequently, in MTPResponder::getObjectPropListReq() iteration over the
items obtained from getObjectPropsSupportedByType() could lead to a null
pointer returned from PropertyPod::getObjectPropDesc() and its
subsequent dereference on the next line of code.