shared/: Please compiler when passed certain compile flags

Fixes the following warn/error when using combination of flags like
building with debug, when disabling optimization and/or when enabling ASAN:

../shared/option-parser.c:61:1: error: control reaches end of non-void function [-Werror=return-type]

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

shared/option-parser.c

@@ -57,6 +57,7 @@ handle_option(const struct weston_option *option, char *value)
 		return true;
 	default:
 		assert(0);
+		return false;
 	}
 }