[lldb][MinidumpFileBuilder] Fix addition of MemoryList steam (#88564)

Summary:
AddMemoryList() was returning the last error status returned by
ReadMemory(). So if an invalid memory region was read last, the function
would return an error.

Test Plan:
./bin/llvm-lit -sv
~/src/llvm-project/lldb/test/API/functionalities/process_save_core_minidump/TestProcessSaveCoreMinidump.py

Reviewers:
kevinfrei,clayborg 

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