glsl: initialise pointer to NULL

Could cause issues if you tried to read from an uninitialised pointer.
This just initalises the pointer to null to avoid that being a problem.
Discovered by Coverity.

CID: 1343616

Signed-off-by: Jakob Sinclair <sinclair.jakob@openmailbox.org>
Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/compiler/glsl/glsl_parser_extras.cpp

@@ -1603,6 +1603,7 @@ ast_struct_specifier::ast_struct_specifier(const char *identifier,
    name = identifier;
    this->declarations.push_degenerate_list_at_head(&declarator_list->link);
    is_declaration = true;
+   layout = NULL;
 }
 
 void ast_subroutine_list::print(void) const