don't attempt with a blank host name in fetch-lua

After exhausting the three URL's for fetch-lua, exit with error,
don't try to connect to a URL with a blank host name.

sys/unix/Makefile.top

@@ -344,7 +344,8 @@ dofiles-nodlb:
 LUA_URL :=www.lua.org/ftp
 LUA_URL_MIRROR :=www.tecgraf.puc-rio.br/lua/mirror/ftp
 LUA_URL_NHD :=www.nethack.org/download/thirdparty
-LUA_URL_list:=$(LUA_URL) $(LUA_URL_MIRROR) $(LUA_URL_NHD)
+LUA_CEASE_TRYING := __cease_trying__
+LUA_URL_list:=$(LUA_URL) $(LUA_URL_MIRROR) $(LUA_URL_NHD) $(LUA_CEASE_TRYING)
 #
 #CURL := curl
 #ask curl to return a failure code if error 404 is encountered
@@ -375,6 +376,7 @@ fetch-Lua:
 	  mkdir -p lib && cd lib && \
 	  while [ $$# -gt 0 ]; do \
 	    shift; \
+	    if [ "$$1" != "$(LUA_CEASE_TRYING)" ]; then \
 	      if [ $$curlstatus -ne 0 ]; then \
 	        luaurl=https://$$1/$$luafile; \
 	        echo Trying $$luaurl; \
@@ -396,6 +398,9 @@ fetch-Lua:
 	          rm -f $$luafile; \
 	        fi; \
 	      fi; \
+	    else \
+              exit 1; \
+	    fi; \
 	  done; \
 	  true )