BaseTools/VfrCompile: Fix segmentation fault issues

BaseTools/VfrCompile: Fix segmentation fault issues

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=532

(1) Add NULL check before using a pointer.
(2) Use "%s" format string in DebugError function to
    avoid crash caused by incorrect input.

Cc: Bo Chen <chenbo@pdx.edu>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Eric Dong <eric.dong@intel.com>