[lldb][MinidumpFileBuilder] Fix addition of MemoryList steam

[mbucko]

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:

Subscribers:

Tasks:

Tags:

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