Fix memory leak in unit conversion

[ianmtaylor1]

♻️ Current situation

A memory leak exists converting inches to millimeters. It is related to an issue created in OpenMQTTGateway: 1technophile/OpenMQTTGateway#1693

💡 Proposed solution

Avoid nesting calls to str_replace, to allow allocated memory to be properly freed.

⚙️ Release Notes

N/A

[NorthernMan54]

Just looked and this is an issue in the upstream repo - https://github.com/merbanan/rtl_433/blob/8950dc41823f0a34dda8a40789134413545e8b62/src/r_api.c#L733

Going to open this there as well

Give me a day or so, and will release this

Tks very much for your efforts on tracking this down

[ianmtaylor1]

Thank you as well, I'm happy I'm able to contribute to this project that's been very useful for me.