Fix invalid pad length [v3]

[gunnsth]

Addresses #412

• Fix when padLen == len(buf). Allows to clear entire buffer.

[codecov]

Codecov Report

Merging #413 into v3 will decrease coverage by 1.98%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##               v3     #413      +/-   ##
==========================================
- Coverage   61.52%   59.53%   -1.99%     
==========================================
  Files         153      153              
  Lines       27761    27671      -90     
==========================================
- Hits        17079    16475     -604     
- Misses      10099    10825     +726     
+ Partials      583      371     -212

Impacted Files	Coverage Δ
pdf/core/security/crypt/filter_aesv3.go	33.82% <50%> (ø)	⬆️
pdf/model/colorspace.go	23.63% <0%> (-14.12%)	⬇️
pdf/core/crypt.go	65.6% <0%> (-9.91%)	⬇️
pdf/model/annotations.go	13.37% <0%> (-9.74%)	⬇️
pdf/model/appender.go	83.08% <0%> (-7.63%)	⬇️
pdf/model/form.go	60.24% <0%> (-7.58%)	⬇️
pdf/model/page.go	54.52% <0%> (-6.97%)	⬇️
common/logging.go	51.02% <0%> (-6.88%)	⬇️
pdf/model/functions.go	31.34% <0%> (-6.85%)	⬇️
pdf/core/utils.go	42.06% <0%> (-6.44%)	⬇️
... and 11 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f8ac397...e0a57fd. Read the comment docs.

[adrg]

The code looks correct. The padding length could potentially be equal to the size of the buffer.