devmapper_wrapper.go: fix gcc warning

I am getting the following warning from gcc when compiling the daemon:

> # github.com/docker/docker/pkg/devicemapper
> pkg/devicemapper/devmapper_wrapper.go: In function ‘log_cb’:
> pkg/devicemapper/devmapper_wrapper.go:20:2: warning: ignoring return
> value of ‘vasprintf’, declared with attribute warn_unused_result
> [-Wunused-result]
>  vasprintf(&buffer, f, ap);
>  ^

vasprintf(3) man page says if the function returns -1, the buffer is
undefined, so we should not use it. In practice, I assume, this never
happens so we just return.

Introduced by moby#33845 that resulted in
commit 63328c6 ("devicemapper: remove 256 character limit of libdm logs")

Cc: Aleksa Sarai <[email protected]>
Signed-off-by: Kir Kolyshkin <[email protected]>

pkg/devicemapper/devmapper_wrapper.go

@@ -15,10 +15,15 @@ static void log_cb(int level, const char *file, int line, int dm_errno_or_class,
 {
  char *buffer = NULL;
  va_list ap;
+ int ret;
 
  va_start(ap, f);
- vasprintf(&buffer, f, ap);
+ ret = vasprintf(&buffer, f, ap);
  va_end(ap);
+ if (ret < 0) {
+ // memory allocation failed -- should never happen?
+ return;
+ }
 
  DevmapperLogCallback(level, (char *)file, line, dm_errno_or_class, buffer);
  free(buffer);