strikethrough-like plaintext can cause parser crash

[ChristianMurphy]

Some strikethrough-like plain text, for example:

-~~~~;~~~~~~

can cause a crash when parsed with markdown-it 12.0.3 using the default settings

require("markdown-it")("default").render("-~~~~;~~~~~~")

exception

markdown-it/lib/rules_inline/balance_pairs.js:37
      if (opener.marker !== closer.marker) continue;
                 ^

TypeError: Cannot read property 'marker' of undefined

[rlidwka]

Fixed, thanks for reporting.