Fix warning introduced by #6412

SOCKET are defined as unsigned integers, thus always >=0.

src/compat.h

@@ -96,7 +96,7 @@ bool static inline IsSelectableSocket(SOCKET s) {
 #ifdef WIN32
     return true;
 #else
-    return (s >= 0 && s < FD_SETSIZE);
+    return (s < FD_SETSIZE);
 #endif
 }