umash_long.inc: mark dead assignments as expected

[pkhuong]

It's more important to remember to always advance the reader cursor after reading bytes than it is to avoid dead code that's trivially eliminated by the compiler.

Fixes #37

[pkhuong]

Before:

umash$ scan-build clang-20 -march=native+crypto -O3 -c umash.c
scan-build: Using '/usr/bin/clang-20' for static analysis
In file included from umash.c:521:
./umash_long.inc:576:4: warning: Value stored to 'data' is never read [deadcode.DeadStores]
  576 |                         data = (const char *)data + sizeof(y);
      |                         ^      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

After:

umash$ scan-build clang-20 -march=native+crypto -O3 -c umash.c
scan-build: Using '/usr/bin/clang-20' for static analysis
scan-build: Analysis run complete.
scan-build: Removing directory '/tmp/scan-build-2025-08-24-120201-300959-1' because it contains no reports.
scan-build: No bugs found.