Fix code scanning alert no. 67: Multiplication result converted to larger type

[tsteven4]

Fixes https://github.com/GPSBabel/gpsbabel/security/code-scanning/67

To fix the problem, we need to ensure that the multiplication is performed using the larger type (qsizetype) before the result is assigned. This can be achieved by casting one of the operands to qsizetype before performing the multiplication. This way, the multiplication will be done using the larger type, preventing overflow.

• Cast one of the operands (members or size) to qsizetype before the multiplication.
• Update the code on line 698 to perform the multiplication using the larger type.

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[codacy-production]

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Coverage variation	Diff coverage
✅ +0.01%	✅ 100.00%

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	Coverable lines	Covered lines	Coverage
Common ancestor commit (a460fb5)	27548	17225	62.53%
Head commit (c33de6c)	55105 (+27557)	34459 (+17234)	62.53% (+0.01%)

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	Coverable lines	Covered lines	Diff coverage
Pull request (#1392)	1	1	100.00%

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