Fix invalid return expressions in functions returning void

Building Gumbo with GCC 4.8.1 using the flags `-std=c99 -Wpedantic` produces the following warnings:

    src/parser.c:46:3: warning: ISO C forbids 'return' with expression, in function returning void [-Wpedantic]
    src/util.c:39:3: warning: ISO C forbids 'return' with expression, in function returning void [-Wpedantic]

Building with tcc causes a fatal error `Cannot assign void value` for the same 2 lines.

This patch makes the warnings/errors go away.