unix: add overflow check for stdio_count in uv_spawn()

Add a bounds check for stdio_count before the allocation in uv_spawn()
on Unix. Reject negative values and values large enough to overflow the
stdio_count * sizeof(*pipes) multiplication. Uses SIZE_MAX to compute
the upper limit portably across 32-bit and 64-bit platforms.

src/unix/process.c

@@ -993,6 +993,10 @@ int uv_spawn(uv_loop_t* loop,
   process->status = 0;
 
   stdio_count = options->stdio_count;
+  if (stdio_count < 0 || (size_t) stdio_count > SIZE_MAX / sizeof(*pipes)) {
+    err = UV_EINVAL;
+    goto error;
+  }
   if (stdio_count < 3)
     stdio_count = 3;