[SPARK-4715][Core] Make sure tryToAcquire won't return a negative value

ShuffleMemoryManager.tryToAcquire may return a negative value. The unit test demonstrates this bug. It will output `0 did not equal -200 granted is negative`.

Author: zsxwing <zsxwing@gmail.com>

Closes #3575 from zsxwing/SPARK-4715 and squashes the following commits:

a193ae6 [zsxwing] Make sure tryToAcquire won't return a negative value