Fix potential infinite loop in AssertionError.get_stack().
Fix potential infinite loop in AssertionError.get_stack().

If testharness.js did not reside under /resources/, the loop to find
the first frame originating from the harness would never terminate.

Fixes #129.

hallvors/testharness.js,bboyle/testharness.js,ayg/testharness.js,jyasskin/testharness.js,hallvors/testharness.js,jyasskin/testharness.js,zqzhang/testharness.js,RatneshThakur/testharness.js