Fix brown-wall robots in D1 non-editor build

tmap == -1 means no override, but 7b33018c changed the comparison to be
true when given negative inputs.  Since -Wsign-compare is off, this was
not caught by the compiler.

similar/main/gamesave.cpp

@@ -178,6 +178,8 @@ static int convert_wclip(int wc) {
 }
 int convert_tmap(int tmap)
 {
+	if (tmap == -1)
+		return tmap;
     return (tmap >= NumTextures) ? tmap % NumTextures : tmap;
 }
 static int convert_polymod(int polymod) {