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.

lauxlib.c

@@ -1025,9 +1025,14 @@ static void *l_alloc (void *ud, void *ptr, size_t osize, size_t nsize) {
 }
 
 
+/*
+** Standard panic funcion just prints an error message. The test
+** with 'lua_type' avoids possible memory errors in 'lua_tostring'.
+*/
 static int panic (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";
   lua_writestringerror("PANIC: unprotected error in call to Lua API (%s)\n",
                         msg);
   return 0;  /* return to Lua to abort */

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 */

manual/manual.of

@@ -4486,6 +4486,10 @@ This string always has a zero (@Char{\0})
 after its last character (as @N{in C}),
 but can contain other zeros in its body.
 
+This function can raise memory errors only
+when converting a number to a string
+(as then it has to create a new string).
+
 }
 
 @APIEntry{lua_Number lua_tonumber (lua_State *L, int index);|