[analyzer] strncpy: Special-case a length of 0 to avoid an incorrect warning.

We check the address of the last element accessed, but with 0 calculating that
address results in element -1. This patch bails out early (and avoids a bunch
of other work at that).

Fixes PR12807.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@156769 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed