Don't cast the return value of malloc/realloc

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

// Don't cast the return value of malloc/realloc.
//
// Casting the return value of malloc/realloc only stands to hide
// errors.

@@
type T;
expression E1, E2;
@@
- (T)
(
_mesa_align_calloc(E1, E2)
|
_mesa_align_malloc(E1, E2)
|
calloc(E1, E2)
|
malloc(E1)
|
realloc(E1, E2)
)

src/mesa/main/extensions.c

@@ -867,7 +867,7 @@ _mesa_make_extension_string(struct gl_context *ctx)
    if (extra_extensions != NULL)
       length += 1 + strlen(extra_extensions); /* +1 for space */
 
-   exts = (char *) calloc(ALIGN(length + 1, 4), sizeof(char));
+   exts = calloc(ALIGN(length + 1, 4), sizeof(char));
    if (exts == NULL) {
       free(extra_extensions);
       return NULL;