Motorola SREC IO plugin: fix multiple bugs

[antonangeloff]

Your checklist for this pull request

• I've read the guidelines for contributing to this repository.
• I made sure to follow the project's coding style.
• I've documented every RZ_API function and struct this PR changes.
• I've added tests that prove my changes are effective (required for changes to RZ_API).
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Detailed description

Fix for a couple of bugs:

• Invalid checksum calculation when writing a new SREC file - as a result the file cannot be opened later by the IO plugin
• Incorrect calculation of the size of the last S-record - writing 1 byte less than it should
• Moved a misplaced EOF check that caused the parser to exit after the first S3 (32-bit address) record is parsed (remaining records get ignored)

Test plan

Added an integration test + changes in some SREC files

Closing issues

None

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 46.77%. Comparing base (5733f05) to head (5561dd2).
⚠️ Report is 2 commits behind head on dev.

Files with missing lines	Patch %	Lines
librz/io/p/io_srec.c	50.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

Files with missing lines	Coverage Δ
librz/io/p/io_srec.c	55.23% <50.00%> (+8.36%)	⬆️

... and 9 files with indirect coverage changes

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