tombstoned: don't increment num_concurrent_dumps until success.

Previously, we would increment num_concurrent_dumps and fail to
decrement it if we failed to start the request. Change this to
only increment after we've successfully started the dump.

Bug: http://b/36685795
Test: debuggerd_test
Change-Id: I66169ed56ed44271e1d8fe1298d95260be7a32a3
(cherry picked from commit 13078245)