Remove useless checks for NULL before freeing

This patch has been generated by the following Coccinelle semantic
patch:

// Remove useless checks for NULL before freeing
//
// free (NULL) is a no-op, so there is no need to avoid it

@@
expression E;
@@
+ free (E);
+ E = NULL;
- if (unlikely (E != NULL)) {
-   free(E);
(
-   E = NULL;
|
-   E = 0;
)
   ...
- }

@@
expression E;
type T;
@@
+ free ((T) E);
+ E = NULL;
- if (unlikely (E != NULL)) {
-   free((T) E);
(
-   E = NULL;
|
-   E = 0;
)
   ...
- }

@@
expression E;
@@
+ free (E);
- if (unlikely (E != NULL)) {
-   free (E);
- }

@@
expression E;
type T;
@@
+ free ((T) E);
- if (unlikely (E != NULL)) {
-   free ((T) E);
- }

Reviewed-by: Brian Paul <brianp@vmware.com>

src/mesa/main/context.c

@@ -1158,11 +1158,9 @@ _mesa_free_context_data( struct gl_context *ctx )
 
    _mesa_free_errors_data(ctx);
 
-   if (ctx->Extensions.String)
-      free((void *) ctx->Extensions.String);
+   free((void *)ctx->Extensions.String);
 
-   if (ctx->VersionString)
-      free(ctx->VersionString);
+   free(ctx->VersionString);
 
    /* unbind the context if it's currently bound */
    if (ctx == _mesa_get_current_context()) {