physicalplan: debugging for segfault in fakeSpanResolverIterator.Seek

Break up a line that is segfaulting into several lines, so that we can tell
which part is to blame if it happens again.

Informs: cockroachdb#100051
Informs: cockroachdb#100108

Epic: None

Release note: None

pkg/sql/physicalplan/fake_span_resolver.go

@@ -168,10 +168,13 @@ func (fit *fakeSpanResolverIterator) Seek(
 	// replicas.
 	fit.ranges = make([]fakeRange, len(splits)-1)
 	for i := range fit.ranges {
+		nodes := fit.fsr.nodes
+		n := len(nodes)
+		j := fit.rng.Intn(n)
 		fit.ranges[i] = fakeRange{
 			startKey: splits[i],
 			endKey:   splits[i+1],
-			replica:  fit.fsr.nodes[fit.rng.Intn(len(fit.fsr.nodes))],
+			replica:  nodes[j],
 		}
 	}