Avoid calling 'fprintf' with NULL

Avoid undefined behavior in calls like «fprintf("%s", NULL)».
('lua_writestringerror' is implemented as 'fprintf', and 'lua_tostring'
can return NULL if object is not a string.)

lauxlib.c

@@ -995,8 +995,10 @@ static void *l_alloc (void *ud, void *ptr, size_t osize, size_t nsize) {
 
 
 static int panic (lua_State *L) {
+  const char *msg = lua_tostring(L, -1);
+  if (msg == NULL) msg = "error object is not a string";
   lua_writestringerror("PANIC: unprotected error in call to Lua API (%s)\n",
-                        lua_tostring(L, -1));
+                        msg);
   return 0;  /* return to Lua to abort */
 }
 

ldblib.c

@@ -417,7 +417,7 @@ static int db_debug (lua_State *L) {
       return 0;
     if (luaL_loadbuffer(L, buffer, strlen(buffer), "=(debug command)") ||
         lua_pcall(L, 0, 0, 0))
-      lua_writestringerror("%s\n", lua_tostring(L, -1));
+      lua_writestringerror("%s\n", luaL_tolstring(L, -1, NULL));
     lua_settop(L, 0);  /* remove eventual returns */
   }
 }

ltests.c

@@ -73,8 +73,10 @@ static void badexit (const char *fmt, const char *s1, const char *s2) {
 
 
 static int tpanic (lua_State *L) {
+  const char *msg = lua_tostring(L, -1);
+  if (msg == NULL) msg = "error object is not a string";
   return (badexit("PANIC: unprotected error in call to Lua API (%s)\n",
-                   lua_tostring(L, -1), NULL),
+                   msg, NULL),
           0);  /* do not return to Lua */
 }