Fix IndexError in scan_tag() and check_key() on empty input

[veeceey]

Summary

Fixes #906 by adding buffer bounds checking before peek(1) calls in scanner methods.

When the input is an empty string or very short, calling peek(1) without validating buffer bounds causes IndexError because the buffer only contains the EOF marker (\0) at position 0, but peek(1) tries to access position 1.

Changes

Added bounds checking in three methods in lib/yaml/scanner.py:

1. check_key() - Check if pointer + 1 < len(buffer) before peek(1)
2. check_value() - Check if pointer + 1 < len(buffer) before peek(1)
3. scan_tag() - Check buffer bounds before both peek(1) and peek() calls

When buffer bounds are insufficient, the code falls back to peek() which safely returns the EOF marker \0.

Testing

Verified the fix with multiple test cases:

# Previously raised IndexError, now works correctly
loader = yaml.loader.SafeLoader("")
loader.check_key()    # Returns True
loader.scan_tag()     # Returns TagToken
loader.check_value()  # Returns True

All existing tests pass:

• tests/test_dump_load.py - 3 tests passed
• tests/legacy_tests/test_errors.py - 189 tests passed

Manual Test Results

1. check_key() with empty input: SUCCESS (no IndexError)
2. scan_tag() with empty input: SUCCESS (no IndexError)  
3. check_value() with empty input: SUCCESS (no IndexError)
4. Various YAML inputs parse correctly without IndexError

The fix ensures proper handling of edge cases while maintaining backward compatibility with all existing functionality.

[veeceey]

Friendly ping - any chance someone could take a look at this when they get a chance? Happy to make any changes if needed.