memblock: return NULL on virtual alloc failure

vm/memblock.c

@@ -267,7 +267,12 @@ void *memblock_alloc(size_t size)
 		memblock_double_capacity(&memblock.reserved);
 	}
 
-	return (void *)(do_alloc(size) + memblock.m_voffset);
+	phys_addr_t p = do_alloc(size);
+	if (p) {
+		p += memblock.m_voffset;
+	}
+
+	return (void *)p;
 }
 
 phys_addr_t memblock_alloc_phys(size_t size)