LibC: Fix bad error check after open() in dlopen()

Author: https://github.com/awesomekling
Commit: https://github.com/SerenityOS/serenity/commit/d71bfb96142

Userland/Libraries/LibC/dlfcn.cpp

@@ -78,7 +78,7 @@ void* dlopen(const char* filename, int flags)
     }
 
     int fd = open(filename, O_RDONLY);
-    if (!fd) {
+    if (fd < 0) {
         g_dlerror_msg = String::formatted("Unable to open file {}", filename);
         return nullptr;
     }