disas/arm: Avoid unintended sign extension

disas/arm: Avoid unintended sign extension

When assembling 'given' from the instruction bytes, C's integer
promotion rules mean we may promote an unsigned char to a signed
integer before shifting it, and then sign extend to a 64-bit long,
which can set the high bits of the long.  The code doesn't in fact
care about the high bits if the long is 64 bits, but this is
surprising, so don't do it.

(Spotted by Coverity, CID 1005404.)

Signed-off-by: Peter Maydell <[email protected]>
Message-id: 1488556233[email protected]