Bugzilla Bug 351609: test lib->dlh before calling NSUnLinkModule on Mac.

r=Mark Mentovai

nsprpub/pr/src/linking/prlink.c

@@ -1279,6 +1279,7 @@ PR_UnloadLibrary(PRLibrary *lib)
 #elif defined(USE_HPSHL)
     result = shl_unload(lib->dlh);
 #elif defined(USE_MACH_DYLD)
+    if (lib->dlh)
         result = NSUnLinkModule(lib->dlh, NSUNLINKMODULE_OPTION_NONE) ? 0 : -1;
 #else
 #error Configuration error