Fix the infinite loop when an input file lacks EOF

The input scanner can be trapped in an infinite loop if the given input
file does not have EOF (and is not ended with a 'whitespace').

The problem is caused by unget(), which keeps rolling back the scanner
pointer without hitting an EOF at the end of the file. This makes getch()
function keep returning the last character of the file and never ends,
and the effect of advance() is always counteracted by unget().

Test/baseResults/preprocessor.eof_missing.vert.out

@@ -0,0 +1,2 @@
+noEOF
+

Test/preprocessor.eof_missing.vert

@@ -0,0 +1 @@
+noEOF

Test/test-preprocessor-list

@@ -13,3 +13,4 @@ preprocessor.simple.vert
 preprocessor.success_if_parse_would_fail.vert
 preprocessor.defined.vert
 preprocessor.many.endif.vert
+preprocessor.eof_missing.vert