ARB prog: Clean up several memory leaks

As far as I am able to determine via code inspection and using
Valgrind, that should be all of the leaks in the parser.

src/mesa/shader/program_parser.h

@@ -39,6 +39,7 @@ enum asm_type {
 };
 
 struct asm_symbol {
+   struct asm_symbol *next;    /**< List linkage for freeing. */
    const char *name;
    enum asm_type type;
    unsigned attrib_binding;
@@ -134,6 +135,14 @@ struct asm_parser_state {
 
    struct _mesa_symbol_table *st;
 
+   /**
+    * Linked list of symbols
+    *
+    * This list is \b only used when cleaning up compiler state and freeing
+    * memory.
+    */
+   struct asm_symbol *sym;
+
    /**
     * State for the lexer.
     */