opcodes/arm: silence compiler warning about uninitialized variable use

opcodes/arm: silence compiler warning about uninitialized variable use

After this commit:

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

      opcodes/arm: add disassembler styling for arm

Some people were seeing their builds failing with complaints about a
possible uninitialized variable usage.  I previously fixed an instance
of this issue in this commit:

  commit 2df82cd4b459fbc32120e0ad1ce19e26349506fe
  Date:   Tue Nov 1 10:36:59 2022 +0000

      opcodes/arm: silence compiler warning about uninitialized variable use

which did fix the build problems that the sourceware buildbot was
hitting, however, an additional instance of the same problem was
brought to my attention, and that is fixed in this commit.

Where commit 2df82cd4b4 fixed the uninitialized variable problem in
print_mve_unpredictable, this commit fixes the same problem in
print_mve_undefined.

As with the previous commit, I don't believe we could really ever get
an uninitialized variable usage, based on the current usage of the
function, so I have just initialized the reason variable to "??".