Check overflow for memory read

[mininny]

Description

Check for memory read overflow in rvgo/fast implementation.
We can check if r.addr + r.count exceeds uint64.max value so that memory read overflow results in error.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 5 lines in your changes missing coverage. Please review.

Project coverage is 61.80%. Comparing base (bf4e465) to head (9826ad5).

Files with missing lines	Patch %	Lines
rvgo/fast/memory.go	0.00%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
- Coverage   61.87%   61.80%   -0.08%     
==========================================
  Files          27       27              
  Lines        4126     4131       +5     
==========================================
  Hits         2553     2553              
- Misses       1432     1436       +4     
- Partials      141      142       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[refcell]

Can r.count exceed 63 causing underflow? Maybe it makes sense to do the following?

Suggested change

	if r.addr > 1<<64-1-r.count {
	if r.count < 64 && r.addr > 1<<64-1-r.count {

[mininny]

This is done to check if the r.addr + r.count does not exceed uint64 max (1<<64-1). I think bit shift has higher operator precedence than subtraction, so underflow won't happen.

[mininny]

Because address space is circular and wrap around in asterisc, we shouldn't assert the maximum address range. See finding#2.