Fix long-standing memory leak as found by Jerry Nichols

Fix long-standing memory leak as found by Jerry Nichols

bob	Cleartext-Password := "hello"
	Tunnel-Server-Endpoint := 1.2.3.4,
        Tunnel-Server-Endpoint := 1.2.3.5,
        Tunnel-Type            := 3,
        Tunnel-Server-Endpoint := 1.2.3.2,
        Tunnel-Medium-Type     := 1,
        Tunnel-Server-Endpoint := 1.2.3.1

Where there are multiple copies of
the same attribute with a := operator in the 'from' list, and the only
operators in the 'from' list are :=, then all the repeated attributes
after the first and before the last non-repeated attribute are lost. In
the example above that would result in the VALUE_PAIRS :

        Tunnel-Server-Endpoint := 1.2.3.5
        Tunnel-Server-Endpoint := 1.2.3.2

being orphaned.