Fix Broken Build, Check ADVANCE_CHAR

Current build (using gcc (GCC) 4.7.2 20121109 (Red Hat 4.7.2-8)) breaks because of the lack of use of the return of two ADVANCE_CHAR macros. This patch will check those return values, and act accordingly if the return indicates failure. Also is the renaming of the POP_CHAR macro to PEEK_CHAR, which better represents the behavior of the macro, as was expressed in the documentation.