Fix medaka not detecting samtools

[zwets]

This fixes issue #537 with the patch proposed by @vineeth-s in the comments on that issue

[aeiwz]

No actionable findings.
The change correctly handles invalid UTF-8 bytes in later samtools --version output while preserving first-line version parsing. A focused reproduction returns 1.13 successfully.

Notes:

• No regression test covers malformed version output.
• The same fix already landed on master in commit 019a2ab (019a2ab) on December 8, 2025.
• PR #550 is therefore obsolete and currently unmergeable; it should be closed.
• The reported failure mode is confirmed in issue #537 (#537 (comment)).