[analyzer] Fix scan-build's -stats mode.

We were failing to match the output line, which led to us collecting no
stats at all, which led to a divide-by-zero error.

Fixes PR15510.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177084 91177308-0d34-0410-b5e6-96231b3b80d8