Fix using void * for unsigned int.

In the array aa_nodes definition, NULL was used to provide the value
zero, resulting in a compiler warning. Define a new constant NONE.
Compiler warning fixed

Clarify code using the result of an assignment as a condition without
parentheses to silence compiler warning.
Silence compiler warning

In the case of an unfinished quoted string, control reaches the end of
a non-void function without returning a value. Now return value of
yyerror.
Main function of program molauto must be of type int