scorch zap panic if mergeFields() sees unsorted fields

mergeFields depends on the fields from the various segments being
sorted for the fieldsSame comparison to work.

Of note, the 'fieldi > 1' guard skips the 0th field, which should
always be the '_id' field.

index/scorch/segment/zap/merge.go

@@ -755,6 +755,10 @@ func mergeFields(segments []*SegmentBase) (bool, []string) {
 			if len(segment0Fields) != len(fields) || segment0Fields[fieldi] != field {
 				fieldsSame = false
 			}
+
+			if fieldi > 1 && field <= fields[fieldi-1] {
+				panic(fmt.Sprintf("mergeFields on unsorted fields: %#v", fields))
+			}
 		}
 	}