[test] Fix rare failure in LineTable editing test

This test will (rarely) delete enough of the string to trip over empty
line and column ranges. After this fix it should be fine to completely
empty the string.

Author: benlangmuir

Tests/SKSupportTests/SupportPerfTests.swift

@@ -47,7 +47,6 @@ final class SupportPerfTests: PerfTestCase {
   }
 
   func testLineTableSingleCharEditPerf() {
-
     let characters: [Character] = [
       "\t", "\n"
       ] + (32...126).map { Character(UnicodeScalar($0)) }
@@ -72,9 +71,9 @@ final class SupportPerfTests: PerfTestCase {
       self.startMeasuring()
 
       for _ in 1...iterations {
-        let line = (0..<(t.count-1)).randomElement(using: &lcg)!
-        let col = (0 ..< t[line].utf16.count).randomElement(using: &lcg)!
-        let len = Bool.random() ? 1 : 0
+        let line = (0 ..< (t.count-1)).randomElement(using: &lcg) ?? 0
+        let col = (0 ..< t[line].utf16.count).randomElement(using: &lcg) ?? 0
+        let len = t[line].isEmpty ? 0 : Bool.random() ? 1 : 0
         var newText = String(characters.randomElement(using: &lcg)!)
         if len == 1 && Bool.random(using: &lcg) {
           newText = "" // deletion