Fix for ExecutionEngine buglet

[Dale E. Martin]

Hello,
I have discovered a small bug in the ExecutionEngine.  A proposed fix is
attached.

There is a place in the ExecutionEngine where it is not resilient to a
failed allocation.  I believe the circumstances are:
* It is near the end of the buffer that has been allocated for code/data
* It is trying to allocate space for a large global, which is not
found in the globalAddressMap
* It tries to allocate it via "getMemoryForGV", but that fails after looking
at CurBufferPtr, so it returns NULL
* It does not check for a NULL return, and then it calls InitializeMemory
with a NULL destination which gets passed into memcpy
* It segvs before looking at CurBufferPtr to see if memory is exhausted and
retrying with more memory allocated

I don't know of a good way to unit test this but I have a repeatable
failure in some code I'm working on and this patch fixes it.

Let me know if you have more questions, comments, etc.

Thanks,
  Dale
-- 
Dale E. Martin - dale at the-martins.org
http://the-martins.org/~dmartin
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