Fix GH-17900 and GH-8084

Calling the constructor twice has no real world benefit.
Block it to fix these two issues.
We also clean up the constructor code a bit:
- `in_ctor` implies `object` exist.
- We surround the instance check with ZEND_DEBUG to avoid a runtime
  penalty.

Closes GH-17900.
Closes GH-8084.

Author: ndossche

ext/mysqli/mysqli_nonapi.c

@@ -72,7 +72,12 @@ void mysqli_common_connect(INTERNAL_FUNCTION_PARAMETERS, bool is_real_connect, b
 	}
 #endif
 
-	if (getThis() && !ZEND_NUM_ARGS() && in_ctor) {
+	if (UNEXPECTED(in_ctor && (Z_MYSQLI_P(object))->ptr)) {
+		zend_throw_error(NULL, "Cannot call constructor twice");
+		return;
+	}
+
+	if (in_ctor && !ZEND_NUM_ARGS()) {
 		php_mysqli_init(INTERNAL_FUNCTION_PARAM_PASSTHRU, in_ctor);
 		return;
 	}
@@ -85,7 +90,9 @@ void mysqli_common_connect(INTERNAL_FUNCTION_PARAMETERS, bool is_real_connect, b
 		}
 
 		if (object) {
+#if ZEND_DEBUG
 			ZEND_ASSERT(instanceof_function(Z_OBJCE_P(object), mysqli_link_class_entry));
+#endif
 			mysqli_resource = (Z_MYSQLI_P(object))->ptr;
 			if (mysqli_resource && mysqli_resource->ptr) {
 				mysql = (MY_MYSQL*) mysqli_resource->ptr;

ext/mysqli/tests/gh17900.phpt

@@ -0,0 +1,16 @@
+--TEST--
+GH-17900 (Assertion failure ext/mysqli/mysqli_prop.c)
+--EXTENSIONS--
+mysqli
+--FILE--
+<?php
+mysqli_report(MYSQLI_REPORT_OFF);
+$mysqli = new mysqli();
+try {
+    $mysqli->__construct('doesnotexist');
+} catch (Error $e) {
+    echo $e->getMessage(), "\n";
+}
+?>
+--EXPECT--
+Cannot call constructor twice

ext/mysqli/tests/mysqli_incomplete_initialization.phpt

@@ -12,8 +12,8 @@ $mysqli->close();
 
 ?>
 --EXPECTF--
-Fatal error: Uncaught Error: mysqli object is not fully initialized in %s:%d
+Fatal error: Uncaught Error: Cannot call constructor twice in %s:%d
 Stack trace:
-#0 %s(%d): mysqli->close()
+#0 %s(%d): mysqli->__construct('doesnotexist')
 #1 {main}
   thrown in %s on line %d