make clang happy take two

src/zm_monitor.cpp

@@ -222,7 +222,7 @@ bool Monitor::MonitorLink::disconnect()
       }
     }
 
-    if ( shmdt( mem_ptr ) < (void *)0 )
+    if ( shmdt( mem_ptr ) < 0 )
     {
       Debug( 3, "Can't shmdt: %s", strerror(errno) );
       return( false );

src/zmf.cpp

@@ -331,7 +331,7 @@ int main( int argc, char *argv[] )
     Debug( 1, "Got image, writing to %s", path );
 
     FILE *fd = 0;
-    if ( (fd = fopen( path, "w" )) < (void *)0 )
+    if ( (fd = fopen( path, "w" )) == NULL )
     {
       Error( "Can't fopen '%s': %s", path, strerror(errno) );
       exit( -1 );