Properly terminated POSIX register sets with LLDB_INVALID_REGNUM.

RegisterSets are assumed to be terminated by this value.  Loops over
register set values would fail without LLDB_INVALID_REGNUM terminating
the list.  This change adjusts the static check to account for the
size of the register set regnum list being one larger than the expected
valid register set count.

llvm-svn: 210964