glsl2: Clear out profile pointers in _mesa_glsl_release_functions

Otherwise builtin_profiles contains dangling pointers the next time
_mesa_read_profile is called.  I suspect this may fix bugzilla #29847,
but I was never able to reproduce it.
diff --git a/src/glsl/builtin_function.cpp b/src/glsl/builtin_function.cpp
index 0a2b012..944265c 100644
--- a/src/glsl/builtin_function.cpp
+++ b/src/glsl/builtin_function.cpp
@@ -19143,6 +19143,7 @@
 {
    talloc_free(builtin_mem_ctx);
    builtin_mem_ctx = NULL;
+   memset(builtin_profiles, 0, sizeof(builtin_profiles));
 }
 
 static void