cmd/asm: fix another infinite loop in register list parser
The code parsing register lists involves an inner loop on
each range defined by the lo,hi bounds. The condition on
this loop (for lo<=hi) is fragile, because the bounds
are unsigned 16 bits numbers.
In some corner cases, the calculated upper bound is 2^16-1
leading to an infinite loop.
Parsing operand `[):[o-FP` results in:
- an infinite loop for non ARM architectures
- the generation of almost 2^16 errors for the ARM architecture
(which are then ignored)
This CL improves the code in 3 ways:
- bail out early when parsing non R prefixed registers
- make sure the register index is never negative
- make sure the number of iterations is limited by the
maximum size of the range (as a defensive measure).
Fixes #12469
Change-Id: Ib1e7e36fb8ad5a3a52c50fc6219d3cfe2b39cc34
Reviewed-on: https://go-review.googlesource.com/14314 Reviewed-by: Rob Pike <r@golang.org>