ast: Initialize location data in constructor of all ast_node objects.

This prevents using uninitialized data in _msea_glsl_error in some
cases, (including at least 6 piglit tests). Thanks to valgrind for
pointing out the problem!

src/glsl/glsl_parser_extras.cpp

@@ -280,7 +280,9 @@ ast_node::print(void) const
 
 ast_node::ast_node(void)
 {
-   /* empty */
+   this->location.source = 0;
+   this->location.line = 0;
+   this->location.column = 0;
 }