Fix compiler warning.

Reported by Christopher Ng.

src/lj_err.c

@@ -329,12 +329,12 @@ static void err_unwind_win_jit(global_State *g, int errcode)
   memset(&hist, 0, sizeof(hist));
   RtlCaptureContext(&ctx);
   while (1) {
-    uintptr_t frame, base, addr = ctx.CONTEXT_REG_PC;
+    DWORD64 frame, base, addr = ctx.CONTEXT_REG_PC;
     void *hdata;
     PRUNTIME_FUNCTION func = RtlLookupFunctionEntry(addr, &base, &hist);
     if (!func) {  /* Found frame without .pdata: must be JIT-compiled code. */
       ExitNo exitno;
-      uintptr_t stub = lj_trace_unwind(G2J(g), addr - sizeof(MCode), &exitno);
+      uintptr_t stub = lj_trace_unwind(G2J(g), (uintptr_t)(addr - sizeof(MCode)), &exitno);
       if (stub) {  /* Jump to side exit to unwind the trace. */
 	ctx.CONTEXT_REG_PC = stub;
 	G2J(g)->exitcode = errcode;