[analyzer] MallocChecker: Avoid redundant transitions.

Don't generate a checker-tagged node unconditionally on the first
checkDeadSymbols callback when no pointers are tracked.

This is a tiny performance optimization; it may change the behavior slightly
by making Static Analyzer bail out on max-nodes one node later (which is good)
but any test would either break for no good reason or become useless
every time someone sneezes.

Differential Revision: https://reviews.llvm.org/D54013

llvm-svn: 347955