In r268463, I misplaced a return in demangle(), causing the function to
erroneously skip symbols that were not mangled at all. Fix this by
moving the return into the preceding if block.

While here, simplify the code by letting cxa_demangle() allocate the
needed space for the demangled symbol. This also fixes a memory leak,
which would occur whenever cxa_demangle() failed.

Reported by: pgj
MFC after: 3 days