Set __BIONIC_COMPLICATED_NULLNESS for the vsnprintf family

When annotating the netinet directory aosp/2552567, we realize the
argment s for vsnprintf family can be null only if the buffer size is 0.
So we correct them and add some tests to verify our assumption.

Bugs: b/245972273
Test: adb shell
Change-Id: I51063286272be0daee0d7c1453a374b1f5674481
diff --git a/tests/fortify_test.cpp b/tests/fortify_test.cpp
index 4abee67..cc3080d 100644
--- a/tests/fortify_test.cpp
+++ b/tests/fortify_test.cpp
@@ -670,6 +670,10 @@
   ASSERT_FORTIFY(readlinkat(AT_FDCWD, "/dev/null", buf, ct));
 }
 
+TEST(TEST_NAME, snprintf_nullptr_valid) {
+  ASSERT_EQ(10, snprintf(nullptr, 0, "0123456789"));
+}
+
 extern "C" char* __strncat_chk(char*, const char*, size_t, size_t);
 extern "C" char* __strcat_chk(char*, const char*, size_t);