mn10300 assembler crashes

[Alexandre Oliva]

The latest patch I installed in the mn10300 assembler was buggy.
Originally, my patch wouldn't free reloc->sym_ptr_ptr, but Nick
correctly suggested me to do so.  I followed his suggestion blindly,
and apparently failed to test the result, because it crashed while
building a mn10300-elf toolchain.

Here's a patch that fixes the crash, that I'm checking in under the
``obviously correct'' rule.

I apologize for the mistake.