jsonblob: check for error in Next

[RTann]

After I thought #1180 would fix the panic I ran into, it turns out it's still there. I believe this is the culprit.

Next does the following loop:

if l.err != nil {
	return false
}

var vs []claircore.Vulnerability
for l.err = l.dec.Decode(&l.de); l.err == nil; l.err = l.dec.Decode(&l.de) { ... }
l.e = l.next
return true

After the loop, it is assumed l.err == nil, but that's not necessarily true. It's possible Decode returns a non-nil error, which should make Next return false.

[codecov]

Codecov Report

Attention: 3 lines in your changes are missing coverage. Please review.

Comparison is base (07ae3f0) 52.23% compared to head (b55b68d) 52.19%.

Files	Patch %	Lines
libvuln/jsonblob/jsonblob.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1181      +/-   ##
==========================================
- Coverage   52.23%   52.19%   -0.05%     
==========================================
  Files         221      221              
  Lines       16878    16881       +3     
==========================================
- Hits         8817     8811       -6     
- Misses       7252     7259       +7     
- Partials      809      811       +2     

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[RTann]

this may not be completely correct. This allows EOF to still return true which is probably ok, but only if l.next != nil, I think. When l.next = nil and l.err == io.EOF, this returns true, but will still cause a panic because l.e will, therefore, be nil, which is never expected when Next returns true

[hdonnay]

By my thinking, the error check on the first entry should catch this?