select.h: make socket validation macros test for INVALID_SOCKET

With Winsock the valid range is [0..INVALID_SOCKET-1] according to
https://docs.microsoft.com/en-us/windows/win32/winsock/socket-data-type-2

Reviewed-by: Jay Satiro
Reviewed-by: Marcel Raad
Reviewed-by: Daniel Stenberg

Closes #5760

lib/select.h

@@ -94,12 +94,23 @@ int tpf_select_libcurl(int maxfds, fd_set* reads, fd_set* writes,
                        fd_set* excepts, struct timeval *tv);
 #endif
 
-/* Winsock and TPF sockets are not in range [0..FD_SETSIZE-1], which
+/* TPF sockets are not in range [0..FD_SETSIZE-1], which
    unfortunately makes it impossible for us to easily check if they're valid
+
+   With Winsock the valid range is [0..INVALID_SOCKET-1] according to
+   https://docs.microsoft.com/en-us/windows/win32/winsock/socket-data-type-2
 */
-#if defined(USE_WINSOCK) || defined(TPF)
+#if defined(TPF)
 #define VALID_SOCK(x) 1
 #define VERIFY_SOCK(x) Curl_nop_stmt
+#elif defined(USE_WINSOCK)
+#define VALID_SOCK(s) ((s) < INVALID_SOCKET)
+#define VERIFY_SOCK(x) do { \
+  if(!VALID_SOCK(x)) { \
+    SET_SOCKERRNO(WSAEINVAL); \
+    return -1; \
+  } \
+} while(0)
 #else
 #define VALID_SOCK(s) (((s) >= 0) && ((s) < FD_SETSIZE))
 #define VERIFY_SOCK(x) do { \