[flang][runtime] Fix octal input of REAL(10) (#74658)

The overflow check didn't work for a 27-digit octal field containing an
80-bit x87 extended precision real value. The count of bytes required
was too large because the leading digit (1) was assumed to require a
full three bits. And the actual transmission of the octal input digits
to the output buffer was incorrect, too.

Fixes llvm-test-suite/Fortran/gfortran/regression/boz_15.f90.