opcodes/arm: silence compiler warning about uninitialized variable use

opcodes/arm: silence compiler warning about uninitialized variable use

The earlier commit:

  commit 6576bffe6cbbb53c5756b2fccd2593ba69b74cdf
  Date:   Thu Jul 7 13:43:45 2022 +0100

      opcodes/arm: add disassembler styling for arm

was causing a compiler warning about a possible uninitialized variable
usage within opcodes/arm-dis.c.

The problem is in print_mve_unpredictable, and relates to the reason
variable, which is set by a switch table.

Currently the switch table does cover every valid value, though there
is no default case.  The variable switched on is passed in as an
argument to the print_mve_unpredictable function.

Looking at how print_mve_unpredictable is used, there is only one use,
the second argument is the one that is used for the switch table,
looking at how this argument is set, I don't believe it is possible
for this argument to take an invalid value.

So, I think the compiler warning is a false positive.  As such, my
proposed solution is to initialize the reason variable to the string
"??", this will silence the warning, and the "??" string should never
end up being printed.