Avoid underflows in 'luaF_getlocalname'

A call like 'debug.getlocal(test, -2147483648)' could decrement
-2147483648, causing an underflow. (As this happens in a debug function,
the fix was not strictly necessary, but it has no cost in the generated
code.)

ldebug.c

@@ -202,7 +202,7 @@ const char *luaG_findlocal (lua_State *L, CallInfo *ci, int n, StkId *pos) {
     if (n < 0)  /* access to vararg values? */
       return findvararg(ci, n, pos);
     else
-      name = luaF_getlocalname(ci_func(ci)->p, n, currentpc(ci));
+      name = luaF_getlocalname(ci_func(ci)->p, cast_uint(n), currentpc(ci));
   }
   if (name == NULL) {  /* no 'standard' name? */
     StkId limit = (ci == L->ci) ? L->top.p : ci->next->func.p;
@@ -226,7 +226,7 @@ LUA_API const char *lua_getlocal (lua_State *L, const lua_Debug *ar, int n) {
     if (!isLfunction(s2v(L->top.p - 1)))  /* not a Lua function? */
       name = NULL;
     else  /* consider live variables at function start (parameters) */
-      name = luaF_getlocalname(clLvalue(s2v(L->top.p - 1))->p, n, 0);
+      name = luaF_getlocalname(clLvalue(s2v(L->top.p - 1))->p, cast_uint(n), 0);
   }
   else {  /* active function; get information through 'ar' */
     StkId pos = NULL;  /* to avoid warnings */
@@ -505,7 +505,7 @@ static const char *kname (const Proto *p, int index, const char **name) {
 static const char *basicgetobjname (const Proto *p, int *ppc, int reg,
                                     const char **name) {
   int pc = *ppc;
-  *name = luaF_getlocalname(p, reg + 1, pc);
+  *name = luaF_getlocalname(p, cast_uint(reg + 1), pc);
   if (*name)  /* is a local? */
     return strlocal;
   /* else try symbolic execution */

lfunc.c

@@ -300,7 +300,7 @@ void luaF_freeproto (lua_State *L, Proto *f) {
 ** Look for n-th local variable at line 'line' in function 'func'.
 ** Returns NULL if not found.
 */
-const char *luaF_getlocalname (const Proto *f, int local_number, int pc) {
+const char *luaF_getlocalname (const Proto *f, unsigned local_number, int pc) {
   int i;
   for (i = 0; i<f->sizelocvars && f->locvars[i].startpc <= pc; i++) {
     if (pc < f->locvars[i].endpc) {  /* is variable active? */

lfunc.h

@@ -58,8 +58,8 @@ LUAI_FUNC StkId luaF_close (lua_State *L, StkId level, TStatus status, int yy);
 LUAI_FUNC void luaF_unlinkupval (UpVal *uv);
 LUAI_FUNC lu_mem luaF_protosize (Proto *p);
 LUAI_FUNC void luaF_freeproto (lua_State *L, Proto *f);
-LUAI_FUNC const char *luaF_getlocalname (const Proto *func, int local_number,
-                                         int pc);
+LUAI_FUNC const char *luaF_getlocalname (const Proto *func,
+                                         unsigned local_number, int pc);
 
 
 #endif

ltests.c

@@ -853,7 +853,7 @@ static int listlocals (lua_State *L) {
   luaL_argcheck(L, lua_isfunction(L, 1) && !lua_iscfunction(L, 1),
                  1, "Lua function expected");
   p = getproto(obj_at(L, 1));
-  while ((name = luaF_getlocalname(p, ++i, pc)) != NULL)
+  while ((name = luaF_getlocalname(p, cast_uint(++i), pc)) != NULL)
     lua_pushstring(L, name);
   return i-1;
 }

testes/db.lua

@@ -258,6 +258,7 @@ print'+'
 -- invalid levels in [gs]etlocal
 assert(not pcall(debug.getlocal, 20, 1))
 assert(not pcall(debug.setlocal, -1, 1, 10))
+assert(not debug.getlocal(test, -2147483648))
 
 
 -- parameter names