Fix potential NULL deref

Backported from phpGH-18697.

Author: ndossche

ext/soap/php_schema.c

@@ -1872,7 +1872,7 @@ static int schema_attribute(sdlPtr sdl, xmlAttrPtr tns, xmlNodePtr attrType, sdl
 		} else {
 			xmlNsPtr nsPtr = attr_find_ns(attr);
 
-			if (strncmp((char*)nsPtr->href, SCHEMA_NAMESPACE, sizeof(SCHEMA_NAMESPACE))) {
+			if (nsPtr && strncmp((char*)nsPtr->href, SCHEMA_NAMESPACE, sizeof(SCHEMA_NAMESPACE))) {
 				smart_str key2 = {0};
 				sdlExtraAttributePtr ext;
 				xmlNsPtr nsptr;