Fix default fromIndex in String#lastIndexOf (AssemblyScript#165)

Author: MaxGraey

std/assembly/string.ts

@@ -229,17 +229,17 @@ export class String {
     return -1;
   }
 
-  lastIndexOf(searchString: String, fromIndex: i32 = 0): i32 {
+  lastIndexOf(searchString: String, fromIndex: i32 = i32.MAX_VALUE): i32 {
     assert(this !== null);
     if (searchString === null) searchString = changetype<String>("null");
     var len: isize = this.length;
     var searchLen: isize = searchString.length;
     if (!searchLen) return len;
     if (!len) return -1;
-    var start = clamp<isize>(fromIndex - searchLen, 0, len);
+    var start = clamp<isize>(fromIndex, 0, len - searchLen);
 
     // TODO: multiple char codes
-    for (let k: isize = len - 1; k >= start; --k) {
+    for (let k = start; k >= 0; --k) {
       if (!compare_memory(
         changetype<usize>(this) + HEADER_SIZE + (k << 1),
         changetype<usize>(searchString) + HEADER_SIZE,