LinkSyntax doesn't parse incorrect link syntax well

[tomyeh]

Parsing 5 Ethernet ([Music]( will cause the following exception:

RangeError (index): Invalid value: Not in range 0..19, inclusive: 20
#0      String.codeUnitAt (dart:core/runtime/libstring_patch.dart:1239:55)
dart-lang/markdown#1      InlineParser.charAt (package:markd/src/inline_parser.dart:177:35)
dart-lang/markdown#2      _moveThroughWhitespace (package:markd/src/inline_parser.dart:999:25)
dart-lang/markdown#3      _parseInlineLink (package:markd/src/inline_parser.dart:857:5)
dart-lang/markdown#4      LinkSyntax.onMatchEnd (package:markd/src/inline_parser.dart:697:24)
dart-lang/markdown#5      TagState.close (package:markd/src/inline_parser.dart:1252:16)
dart-lang/markdown#6      TagState.tryMatch (package:markd/src/inline_parser.dart:1191:7)
dart-lang/markdown#7      InlineParser.parse.<anonymous closure> (package:markd/src/inline_parser.dart:163:57)
dart-lang/markdown#8      ListIterable.any (dart:_internal/iterable.dart:89:15)
dart-lang/markdown#9      InlineParser.parse (package:markd/src/inline_parser.dart:163:12)
dart-lang/markdown#10     Document.parseInline (package:markd/src/document.dart:71:64)
dart-lang/markdown#11     Document._parseInlineContent (package:markd/src/document.dart:77:27)
dart-lang/markdown#12     Document._parseInlineContent (package:markd/src/document.dart:82:9)
dart-lang/markdown#13     Document.parseLines (package:markd/src/document.dart:66:5)
dart-lang/markdown#14     markdownToHtml (package:markd/src/html_renderer.dart:33:32)
dart-lang/markdown#15     main (file:///Users/tomyeh/prj/r/markd/test/markdown_test.dart:11:7)