Panic functions should not raise errors

The standard panic function was using 'lua_tostring', which may raise
a memory-allocation error if error value is a number.

ltests.c

@@ -73,8 +73,9 @@ 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";
+  const char *msg = (lua_type(L, -1) == LUA_TSTRING)
+                  ? lua_tostring(L, -1)
+                  : "error object is not a string";
   return (badexit("PANIC: unprotected error in call to Lua API (%s)\n",
                    msg, NULL),
           0);  /* do not return to Lua */