glsl2: Initialize location structure at beginning of parse.

Since we have a custom structure for YYLTYPE locations, we need to use
an %initial-action directive to avoid triggering use of uninitialized
memory when, for example, printing error messages.

Thanks to valgrind for noticing this bug.

src/glsl/glsl_parser.ypp

@@ -35,9 +35,17 @@
 %}
 
 %pure-parser
-%locations
 %error-verbose
 
+%locations
+%initial-action {
+   @$.first_line = 1;
+   @$.first_column = 1;
+   @$.last_line = 1;
+   @$.last_column = 1;
+   @$.source = 0;
+}
+
 %lex-param   {void *scanner}
 %parse-param {struct _mesa_glsl_parse_state *state}
 %name-prefix "_mesa_glsl_"