[libFuzzer] Add dummy call of LLVMFuzzerTestOneInput to afl_driver.

Summary:
Add dummy call of LLVMFuzzerTestOneInput to afl_driver before it starts
executing on actual inputs. Do this so that first time initialization
performed by LLVMFuzzerTestOneInput is not considered code covered by
a particular input.

Patch By: metzman

Reviewers: kcc, morehouse

Reviewed By: kcc

Subscribers: llvm-commits, Sanitizers

Differential Revision: https://reviews.llvm.org/D41193

llvm-svn: 320643