blobstore: workaround for bs_cluster_to_lba() scan-build false positive

Patch https://review.spdk.io/gerrit/c/spdk/spdk/+/16328,
introduced a refactor for persist path. No functional change
should occur with it, but the code layout after compilation
(path length) might have changed.

It resulted in unrelated scan-build failure:
https://ci.spdk.io/results/autotest-per-patch/builds/95746/archive/scanbuild-vg-autotest/scan-build/report-d08e76.html#EndPath

Tried to replicate the issue without the above patch,
by increasing maxloop or -analyze-headers in scan-build.
Didn't result in any new failures in blobstore.

This seemed like a false positive, so it was verified:
https://review.spdk.io/gerrit/c/spdk/spdk/+/16333/

With no other options, an assert is added only to the
function where the false positive occured.

scan-build log for posterity:
blobstore.c:1062:58: warning: Division by zero [core.DivideZero]
                desc_extent_rle->extents[extent_idx].cluster_idx = lba /
lba_per_cluster;
                                                                   ~~~~^~~~~~~~~~~~~~~~~
blobstore.c:1079:58: warning: Division by zero [core.DivideZero]
                desc_extent_rle->extents[extent_idx].cluster_idx = lba /
lba_per_cluster;
                                                                   ~~~~^~~~~~~~~~~~~~~~~

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: I9dd729fa13ce1c9bbcb91e4326658e2b4e326e6d
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/16335
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@nvidia.com>
Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>

lib/blob/blobstore.c

@@ -1045,6 +1045,8 @@ blob_serialize_extent_rle(const struct spdk_blob *blob,
 	desc_extent_rle->type = SPDK_MD_DESCRIPTOR_TYPE_EXTENT_RLE;
 
 	lba_per_cluster = bs_cluster_to_lba(blob->bs, 1);
+	/* Assert for scan-build false positive */
+	assert(lba_per_cluster > 0);
 
 	lba = blob->active.clusters[start_cluster];
 	lba_count = lba_per_cluster;