Move stack->next assignment within if (stack) check

The code checks if stack is a NULL pointer. Below that if the
stack->next pointer is updated unconditionally. Therefore a call with a
NULL pointer will crash, even though the if (stack) check shows the
intent that it is valid to call the function with NULL. The assignment
to stack->next was not in the initial version of the code.
Fix it by moving the assignment inside the if's body.

Author: nielsdos

sapi/phpdbg/phpdbg_cmd.c

@@ -422,10 +422,9 @@ PHPDBG_API void phpdbg_stack_free(phpdbg_param_t *stack) {
 				remove = next;
 			else break;
 		}
-	}
-
 
-	stack->next = NULL;
+		stack->next = NULL;
+	}
 } /* }}} */
 
 /* {{{ */