Make memory checker happy about FileGlobal

FileGlobal structure in write_global() didn't initialize
the while build_hash property and only placed there null
terminated meaningful content, leaving trailing uninitialized.

This made valgrind complain about using uninitialized memory
later in undo stack.

source/blender/blenloader/intern/writefile.c

@@ -3274,6 +3274,7 @@ static void write_global(WriteData *wd, int fileflags, Main *mainvar)
 	/* prevent mem checkers from complaining */
 	fg.pads= 0;
 	memset(fg.filename, 0, sizeof(fg.filename));
+	memset(fg.build_hash, 0, sizeof(fg.build_hash));
 
 	current_screen_compat(mainvar, &screen);