Ignore clang-analyzer-deadcode.DeadStores in BlobLibrary ctor

This is a dead store which we may want to eliminate. We do read from
this field earlier in the constructor, and the offset should
conceptually be incremented here, but it's never (currently) read after
the conditional + memcpy earlier in the method. I'm assuming it may have
been kept intentionally as potential future-proofing in case of future
use of offset later in the method. This write can safely be removed with
the current code though.

Author: cbracken

impeller/blobcat/blob_library.cc

@@ -42,7 +42,7 @@ BlobLibrary::BlobLibrary(std::shared_ptr<fml::Mapping> mapping)
   {
     const size_t read_size = sizeof(Blob) * header.blob_count;
     ::memcpy(blobs.data(), mapping_->GetMapping() + offset, read_size);
-    offset += read_size;
+    offset += read_size;  // NOLINT(clang-analyzer-deadcode.DeadStores)
   }
 
   // Read the blobs.