glsl: Initialize ast_jump_statement::opt_return_value.

opt_return_value was not initialized if mode != ast_return.

Fixes "Uninitialized pointer field" defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>

src/glsl/glsl_parser_extras.cpp

@@ -1225,6 +1225,7 @@ ast_jump_statement::print(void) const
 
 
 ast_jump_statement::ast_jump_statement(int mode, ast_expression *return_value)
+   : opt_return_value(NULL)
 {
    this->mode = ast_jump_modes(mode);