Merge pull request #2754 from markusbeth/fix-mono-config

fixed replacement of $mono_libdir in config file

strncpy does not NUL-terminate the destination string in this case so one cannot use strcat to append

This fixes the error "malloc: top chunk is corrupt" as reported on mono-devel-list back in 2015-10-20 (subject: "malloc error executing OBS-built mono") at least for me.

diff --cc mono/metadata/assembly.c
Simple merge