Fix possible crash in case of exception

Fixes oss-fuzz #49068

Author: dstogov

ext/reflection/php_reflection.c

@@ -6972,6 +6972,9 @@ ZEND_METHOD(ReflectionEnumBackedCase, getBackingValue)
 
 	if (Z_TYPE(ref->value) == IS_CONSTANT_AST) {
 		zval_update_constant_ex(&ref->value, ref->ce);
+		if (EG(exception)) {
+			return;
+		}
 	}
 
 	ZEND_ASSERT(intern->ce->enum_backing_type != IS_UNDEF);

ext/reflection/tests/ReflectionEnumBackedCase_getBackingValue_002.phpt

@@ -0,0 +1,15 @@
+--TEST--
+ReflectionEnumBackedCase::getBackingValue() exception
+--FILE--
+<?php
+enum Inm:int {
+    case Foo=y;
+}
+try {
+    var_dump((new ReflectionEnumBackedCase(Inm::class,'Foo'))->getBackingValue());
+} catch (Error $e) {
+    echo $e->getMessage() . "\n";
+}
+?>
+--EXPECT--
+Undefined constant "y"