Ensure reuc vector is always valid

In theory, if there was a problem reading the REUC data, the `read_reuc()` routine could have left uninitialized and invalid data in the `git_index` vector.  This moves the line that inserts a new entry into the vector down to the bottom of the routine so we know all the content is already valid.  Also, per @linquize, this uses `calloc` to ensure no uninitialized data.