Fix memory allocation failed regression introduced by #22766 (#23776)

Author: ibuclaw

compiler/src/dmd/dinterpret.d

@@ -2907,7 +2907,8 @@ public:
             for (ClassDeclaration c = cd; c; c = c.baseClass)
                 totalFieldCount += c.fields.length;
 
-            totalFieldCount -= cd.hasMonitor(); // skip __monitor field
+            if (totalFieldCount)
+                totalFieldCount -= cd.hasMonitor(); // skip __monitor field
 
             auto elems = new Expressions(totalFieldCount);
             ptrdiff_t fieldsSoFar = totalFieldCount;

compiler/test/compilable/reg22766.d

@@ -0,0 +1,11 @@
+/*
+TEST_OUTPUT:
+---
+C22766()
+---
+*/
+module object;
+
+class C22766 { }
+
+pragma(msg, new C22766);