Fix position calculations of String.startsWith (#3734)

Fixes two bugs in the index calculations that affect the result
of the `String.startsWith` function:

- Fixes the implementation of ECMA-262 v6, 21.1.3.18, step 14:
  "If searchLength+start is greater than len, return false".
- Fixes the return value of the helper function
  `ecma_builtin_helper_string_find_index`. If it is called with a
  starting search position, the returned index should be
  not less than that.

These changes fix the following test cases in the test262 suite:

- built-ins/String/prototype/startsWith/return-true-if-searchstring-is-empty.js
- built-ins/String/prototype/startsWith/searchstring-found-with-position.js

JerryScript-DCO-1.0-Signed-off-by: Mátyás Mustoha mmatyas@inf.u-szeged.hu

tests/jerry/es2015/string-prototype-startswith.js

@@ -15,7 +15,10 @@
 var x = "My cat is awesome";
 assert (x.startsWith ("My"));
 assert (x.startsWith ("cat", 3));
+assert (x.startsWith ("awesome", 10));
 assert (x.startsWith (""));
+assert (x.startsWith ("", 1));
+assert (x.startsWith ("", 17));
 assert (x.startsWith ([]));
 
 assert (x.startsWith ("doggo") === false);