d3dcompiler: Fix a leak on error path (Coverity).

dlls/d3dcompiler_43/utils.c

@@ -1446,7 +1446,7 @@ static BOOL invert_swizzle(unsigned int *swizzle, unsigned int *writemask, unsig
 struct hlsl_ir_node *add_assignment(struct list *instrs, struct hlsl_ir_node *lhs,
         enum parse_assign_op assign_op, struct hlsl_ir_node *rhs)
 {
-    struct hlsl_ir_assignment *assign = d3dcompiler_alloc(sizeof(*assign));
+    struct hlsl_ir_assignment *assign;
     struct hlsl_type *lhs_type;
     DWORD writemask = 0;
 
@@ -1459,6 +1459,7 @@ struct hlsl_ir_node *add_assignment(struct list *instrs, struct hlsl_ir_node *lh
             return NULL;
     }
 
+    assign = d3dcompiler_alloc(sizeof(*assign));
     if (!assign)
     {
         ERR("Out of memory\n");