txt parser: fix goroutine leak

[miekg]

When a higher level (grammar or syntax) error was encountered the lower
level zlexer routine would be left open. This leaks a goroutine.

This PR fixes this by signalling this error, retrieving any remaining
items from the channel, so zlexer can return.

Fixes #586
Fixes coredns/coredns#1233

[codecov-io]

Codecov Report

Merging #570 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #570      +/-   ##
=========================================
+ Coverage    58.3%   58.4%   +0.09%     
=========================================
  Files          37      37              
  Lines        9865    9883      +18     
=========================================
+ Hits         5752    5772      +20     
+ Misses       3064    3063       -1     
+ Partials     1049    1048       -1

Impacted Files	Coverage Δ
scanner.go	100% <100%> (ø)	⬆️
scan.go	80.48% <100%> (+0.17%)	⬆️
dnssec_keyscan.go	71.26% <100%> (+0.85%)	⬆️
scan_rr.go	70.06% <0%> (+0.14%)	⬆️

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[miekg]

The

_, ok := <-c
 +		_, ok = <-c
 +		if !ok {
 +			// too bad
 +		}

is ugly, but I don't have a better idea. I'm pondering on adding a test (or figuring out one)

[miekg]

Added leaktest:

this branch:

=== RUN   TestParseBadNAPTR
--- PASS: TestParseBadNAPTR (0.01s)
PASS
ok  	github.com/miekg/dns	1.022s

master branch with test copied in:

=== RUN   TestParseBadNAPTR
1 instances of:
github.com/miekg/dns.zlexer(0xc42005c080, 0xc42001c060)
	/home/miek/g/src/github.com/miekg/dns/scan.go:602 +0x52a
created by github.com/miekg/dns.parseZone
	/home/miek/g/src/github.com/miekg/dns/scan.go:185 +0x297
--- FAIL: TestParseBadNAPTR (0.52s)
	parse_test.go:1417: leaked goroutines: too many goroutines running after dns test(s)
FAIL
exit status 1
FAIL	github.com/miekg/dns	0.534s