glsl: Initialize ast_node member field location.path in constructor.

Fix defect reported by Coverity Scan.

Uninitialized pointer field (UNINIT_CTOR)
uninit_member: Non-static class member field location.path is not
initialized in this constructor nor in any functions that it calls.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6905>

src/compiler/glsl/glsl_parser_extras.cpp

@@ -1188,6 +1188,7 @@ ast_node::print(void) const
 
 ast_node::ast_node(void)
 {
+   this->location.path = NULL;
    this->location.source = 0;
    this->location.first_line = 0;
    this->location.first_column = 0;