shared/dissect-image: make sure that we don't truncate device name

gcc-9 complains that the string may be truncated when written into the output
structure. This shouldn't happen, but if it did, in principle we could remove a
different structure (with a matching name prefix). Let's just refuse the
operation if the name doesn't fit.

src/shared/dissect-image.c

@@ -1178,7 +1178,6 @@ int dissected_image_decrypt_interactively(
 
 #if HAVE_LIBCRYPTSETUP
 static int deferred_remove(DecryptedPartition *p) {
-
         struct dm_ioctl dm = {
                 .version = {
                         DM_VERSION_MAJOR,
@@ -1199,6 +1198,9 @@ static int deferred_remove(DecryptedPartition *p) {
         if (fd < 0)
                 return -errno;
 
+        if (strlen(p->name) > sizeof(dm.name))
+                return -ENAMETOOLONG;
+
         strncpy(dm.name, p->name, sizeof(dm.name));
 
         if (ioctl(fd, DM_DEV_REMOVE, &dm))