core: prevent crash in tee_mmu_final() on TA loading error

If the creation of the TA execution context fails before the mapping
directives are initialized, tee_mmu_final() will be called with the TA
context field mmu being NULL.

This change allows tee_mmu_final() to be called with uninitialized
mapping resources without crashing the core.

Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>
Reviewed-by: Jens Wiklander <jens.wiklander@linaro.org>

core/arch/arm/mm/tee_mmu.c

@@ -676,6 +676,9 @@ void tee_mmu_rem_rwmem(struct user_ta_ctx *utc, struct mobj *mobj, vaddr_t va)
  */
 void tee_mmu_final(struct user_ta_ctx *utc)
 {
+	if (!utc->mmu)
+		return;
+
 	/* clear MMU entries to avoid clash when asid is reused */
 	tlbi_asid(utc->mmu->asid);