libmatchbox: Fix the matchbox environment start failure on x86-64 target.
x86 target works well. Root cause is libmatchbox use "0"(int) as termination
indicator when calling XftFontOpen, which in turn called FcPatternVapBuild(in
fontconfig). It try to get the "0" as char* and fetch wrong value, as int and
char* has different size on x86-64. This patch forces a NULL pointer as
terminator to fix it.
