sql: fix early exit from merge index planner

The MergeIndexes() method of the IndexBackfillerMergePlanner has a check
to see if there are any spans to merge before proceeding with building
the plan for the merge. Previously the check for this was checking
whether there were any elements in spansToDo, which is allocated to have
one entry per source index ID, causing the length check to be vacuously
true. The logic probably meant to to check the length of the spans list
for each individual source index ID, which this patch puts into place.

Epic: none
Release note: None

Author: mw5h

pkg/sql/mvcc_backfiller.go

@@ -58,7 +58,7 @@ func (im *IndexBackfillerMergePlanner) MergeIndexes(
 		g.Add(sourceIndexSpan)
 		g.Sub(progress.CompletedSpans[i]...)
 		spansToDo[i] = g.Slice()
-		if len(spansToDo) > 0 {
+		if len(spansToDo[i]) > 0 {
 			hasToDo = true
 		}
 	}