Fix buffer access of a referenced string

[stephaneDunand]

Hello,
I have identified an error with referenced string in the following two methods. Here is the correction I propose (in zend/value.cpp) :
char *Value::buffer() const
{
// must be a string
if (!isString()) return nullptr;

zend_string* zs = zval_get_string(_val);
char* raw = ZSTR_VAL(zs);
zend_string_release(zs);
return raw;

}
const char *Value::rawValue() const
{
// must be a string
if (!isString()) return nullptr;

zend_string* zs = zval_get_string(_val);
char* raw = ZSTR_VAL(zs);
zend_string_release(zs);
return (const char*)raw;

}