Revert "threads.c:6403:6: warning: 'current_idx' may be used uninitialized in this...

Revert "threads.c:6403:6: warning: 'current_idx' may be used uninitialized in this function [-Wmaybe-uninitialized]"

This reverts commit mono/mono@bf2f1abe01978102c22d76b4a10996e8d8b50b0f.

The logic is not clear as to if there is a bug, so leave the warning.
There are two uses of the possibly uninitialized variable.
One is guarded and not a bug.
The other is not clear, and don't want to blindly use 0.

Commit migrated from https://github.com/mono/mono/commit/a97408ac1e9b1e0dedbd67424ca0b0a6fc7b73b6