resolve MSVC warning (#321)

[ArmaanCPro]

This references issue #321.

It was a stupidly simple 1-line solution, but hopefully it'll help projects that build using the latest version of MSVC.

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Resolved the warning on MSVC:
inja\include\inja\parser.hpp(662) : warning C4702: unreachable code.

The statement current_block = nullptr was unreachable after the default case of the switch statement that threw, so I just moved the line up to be inside of the default case.

[villevoutilainen]

Three comments:

1. MSVC is correct, that resetting of current_token after the switch is unreachable, the parser either reaches EOF and returns, or throws
2. None of the other branches that throw an error reset current_token
3. ..and that looks intentional; if the parser throws, the calling code can show the current_token in case of a parser error. So just remove that outside-switch resetting of current_token, and don't move it.