[SV 60798] Silence bogus GCC10 and GCC11 warnings

[SV 60798] Silence bogus GCC10 and GCC11 warnings

* src/main.c (main): Use a separate variable to track final character.
* src/read.c (eval): Track the semicolon position not one beyond it.
* src/variable.c (do_variable_definition): Include a default switch
case to ease the work of the exhaustiveness prover.