io.readLine adds '\00' char to the end

[inv2004]

Description

import strutils

const FILE = "/tmp/failed_file"

let hex = "313632313920313632343720313632353920313632363020313632393020323035363520323037323120323131353020323239393820323331303520323332313020323332343820323332363820"

### buf.len == 78 => error
### if I remove 1 char and buf.len == 77 => everything is ok

writeFile FILE, parseHexStr(hex)

doAssert readFile(FILE).toHex == hex

echo hex

let f = open(FILE)
var s = newString(80)
while f.readLine(s):
  echo s.toHex
  doAssert s.toHex == hex

Nim Version

1.6.10 1.6.8 1.4.8 1.2.8

Current Output

313632313920313632343720313632353920313632363020313632393020323035363520323037323120323131353020323239393820323331303520323332313020323332343820323332363820
31363231392031363234372031363235392031363236302031363239302032303536352032303732312032313135302032323939382032333130352032333231302032333234382032333236382000
/home/u/t.nim(17)        t
/home/u/.choosenim/toolchains/nim-1.6.10/lib/system/assertions.nim(38) failedAssertImpl
/home/u/.choosenim/toolchains/nim-1.6.10/lib/system/assertions.nim(28) raiseAssert
/home/u/.choosenim/toolchains/nim-1.6.10/lib/system/fatal.nim(54) sysFatal
Error: unhandled exception: /home/u/t.nim(17, 12) `s.toHex == hex`  [AssertionDefect]

### Expected Output

```text
313632313920313632343720313632353920313632363020313632393020323035363520323037323120323131353020323239393820323331303520323332313020323332343820323332363820
313632313920313632343720313632353920313632363020313632393020323035363520323037323120323131353020323239393820323331303520323332313020323332343820323332363820

### Possible Solution

_No response_

### Additional Information

_No response_