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.
Merge pull request #1591 from arrbee/reorder-reuc-read-for-safety

Ensure reuc vector is always valid