Handle error case when looking for reg pair

Handle error case when looking for reg pair

When performing unsigned up-cast to long, if needReg was computed
with exactly 2 bits, then rsFindRegPairNo() is called to find the
corresponding reg pair. This can fail if one of the needReg bits
is in the reserved set (rsMaskResvd), as was the case in this test
on ARM for R10.

Add logic to handle this error return. The same logic already existed
in the signed cast case.

Fixes #12886