Fix debuggerd (native crash dump).

Change 44659e90 (6cc49230 in AOSP) introduced walking
both the symbol table and dynamic symbol table. The problem
is that it was dereferencing values whether or not the two tables
were both present, which could wind up reading from invalid memory.
The read from a bad address would cause debuggerd itself to crash,
which isn't handled.

Change-Id: Ie936f660018b1980dee5b6ed669588db861f1a79