avcodec/v4l2_m2m: Avoid using intermediate buffer

Up until now, v4l2_m2m would write via snprintf() into an intermediate
buffer and then copy from there (via strncpy()) to the end buffer. This
commit changes this by removing the intermediate buffer.

The call to strncpy() was actually of the form strncpy(dst, src,
strlen(src) + 1) which is unsafe in general, but safe in this instance
because dst and src were both of the same size and src was a proper
zero-terminated string. But this nevertheless led to a compiler warning
"‘strncpy’ specified bound depends on the length of the source argument
[-Wstringop-overflow=]" in GCC 9.2. strlen() was unnecessary anyway.
Reviewed-by: Andriy Gelman <andriy.gelman@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/v4l2_m2m.c

@@ -358,7 +358,6 @@ int ff_v4l2_m2m_codec_init(V4L2m2mPriv *priv)
 {
     int ret = AVERROR(EINVAL);
     struct dirent *entry;
-    char node[PATH_MAX];
     DIR *dirp;
 
     V4L2m2mContext *s = priv->context;
@@ -372,9 +371,8 @@ int ff_v4l2_m2m_codec_init(V4L2m2mPriv *priv)
         if (strncmp(entry->d_name, "video", 5))
             continue;
 
-        snprintf(node, sizeof(node), "/dev/%s", entry->d_name);
-        av_log(s->avctx, AV_LOG_DEBUG, "probing device %s\n", node);
-        strncpy(s->devname, node, strlen(node) + 1);
+        snprintf(s->devname, sizeof(s->devname), "/dev/%s", entry->d_name);
+        av_log(s->avctx, AV_LOG_DEBUG, "probing device %s\n", s->devname);
         ret = v4l2_probe_driver(s);
         if (!ret)
             break;
@@ -389,7 +387,7 @@ int ff_v4l2_m2m_codec_init(V4L2m2mPriv *priv)
         return ret;
     }
 
-    av_log(s->avctx, AV_LOG_INFO, "Using device %s\n", node);
+    av_log(s->avctx, AV_LOG_INFO, "Using device %s\n", s->devname);
 
     return v4l2_configure_contexts(s);
 }