Remove FATAL from pg_lzdecompress

I attach patch which adds boundaries check and memory overwriting
protection when compressed data are corrupted.

Current behavior let code overwrite a memory and after that check if
unpacked size is same as expected value. In this case elog execution
fails (at least on Solaris - malloc has corrupted structures) and no
message appears in a log file.

I did not add any extra information into the message. Reasonable
solution seems to be use errcontext how was recommended by Alvaro. But I
'm not sure if printtup is good place for it, because pg_detoast is
called from many places. However, is can be solved in separate patch.

I'm also think that this modification should be backported to other
version too.

Thanks Zdenek