Add assertions to satisfy Coverity about possible underflows.

Coverity complained about possible underflows so add assertions to demonstrate that the values in question are greater than zero.

src/command/backup/blockMap.c

@@ -485,7 +485,7 @@ blockMapWrite(const BlockMap *const this, IoWrite *const output, const size_t bl
             {
                 ASSERT(
                     superBlock == blockMapGet(this, blockIdx) ||
-                    blockMapGet(this, blockIdx)->block == blockMapGet(this, blockIdx - 1)->block + 1);
+                    (blockIdx > 0 && (blockMapGet(this, blockIdx)->block == blockMapGet(this, blockIdx - 1)->block + 1)));
 
                 ioWrite(output, BUF(blockMapGet(this, blockIdx)->checksum, checksumSize));
             }

src/common/io/filter/group.c

@@ -206,6 +206,8 @@ ioFilterGroupOpen(IoFilterGroup *const this)
 
             // If this is not the last output filter then create a new output buffer for it. The output buffer for the last filter
             // will be provided to the process function.
+            ASSERT(ioFilterGroupSize(this) != 0);
+
             if (ioFilterOutput(filterData->filter) && filterIdx < ioFilterGroupSize(this) - 1)
             {
                 filterData->output = bufNew(ioBufferSize());