cxx: add nullptr check in calloc

Even though malloc via the cxx_heap never fails, the implementation of
calloc should better not rely on this assumption.

repos/base/src/lib/cxx/malloc_free.cc

@@ -87,7 +87,11 @@ extern "C" void *malloc(size_t size)
 
 extern "C" void *calloc(size_t nmemb, size_t size)
 {
-	void *addr = malloc(nmemb*size);
+	void * const addr = malloc(nmemb*size);
+
+	if (addr == nullptr)
+		return nullptr;
+
 	Genode::memset(addr, 0, nmemb*size);
 	return addr;
 }