When recovery is rolling back an incomplete instant DROP COLUMN
operation, it may access non-existing fields. Let us avoid
invoking std::find_if() outside the valid bounds of the array.

This bug was reproduced with the Random Query Generator, using a
combination of instant DROP, ADD...FIRST, CHANGE (renaming a column).
Unfortunately, we were unable to create an mtr test case for
reproducing this, despite spending considerable effort on it.