[ATM] ambassador, firestream: "-1 >>" is implementation defined

[ATM] ambassador, firestream: "-1 >>" is implementation defined

6.5.7(5): The result of E1 >> E2 is E1 right-shifted E2 bit positions.
            ...
    If E1 has a signed type and a negative value, the resulting value
    is implementation defined.

So, cast -1 to unsigned type to make result well-defined.

[ Modified to use ~0U based upon recommendation from Al Viro. -DaveM ]

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Adrian Bunk <bunk@stusta.de>