clear backtrace when throwing an exception

Without this, the ext/soap tests would trigger a double stack trace:

```
  Fatal error: Uncaught SoapFault exception: [WSDL] SOAP-ERROR: Parsing Schema: can't import schema from '/Users/enorris/workspace/php-src/ex\
t/soap/tests/bugs/bug62900.xsd', unexpected 'targetNamespace'='http://www.w3.org/XML/1998/namespacex', expected 'http://www.w3.org/XML/1998\
/namespace' in /Users/enorris/workspace/php-src/ext/soap/tests/bugs/bug62900_run:2
  #0 /Users/enorris/workspace/php-src/ext/soap/tests/bugs/bug62900_run(2): SoapClient->__construct('/Users/enorris/...')
  #1 {main}
    thrown in /Users/enorris/workspace/php-src/ext/soap/tests/bugs/bug62900_run on line 2
  Stack trace:
  #0 /Users/enorris/workspace/php-src/ext/soap/tests/bugs/bug62900_run(2): SoapClient->__construct('/Users/enorris/...')
  #1 {main}
```

Zend/zend_exceptions.c

@@ -904,6 +904,10 @@ ZEND_API ZEND_COLD zend_result zend_exception_error(zend_object *ex, int severit
 	ZVAL_OBJ(&exception, ex);
 	ce_exception = ex->ce;
 	EG(exception) = NULL;
+
+	zval_ptr_dtor(&EG(error_backtrace));
+	ZVAL_UNDEF(&EG(error_backtrace));
+
 	if (ce_exception == zend_ce_parse_error || ce_exception == zend_ce_compile_error) {
 		zend_string *message = zval_get_string(GET_PROPERTY(&exception, ZEND_STR_MESSAGE));
 		zend_string *file = zval_get_string(GET_PROPERTY_SILENT(&exception, ZEND_STR_FILE));