Fixed warning/error when compiling with `clang -Os`.
Fixed warning/error when compiling with `clang -Os`.

DISCUSSION:

When compiling with `-Os` for "smallest, fastest", an error is reported in `parser.c`:

```
/Users/siegel/git/tree-sitter/lib/src/./parser.c:1368:10: error: unused variable 'did_merge' [-Werror,-Wunused-variable]
    bool did_merge = ts_stack_merge(self->stack, version, previous_version_count);
         ^
1 error generated.
```

This is because with `NDEBUG` set,  `assert(e)` collapses to `(void)0`,
which in turn means that `did_merge` does not actually get consumed.
This seems to get caught when compiling with `-Os`, but not otherwise.

Compiler version:
```
Apple clang version 13.0.0 (clang-1300.0.29.30)
Target: arm64-apple-darwin21.3.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin
```

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