ensure programmer serial number string is properly zero terminated.

[xythobuz]

I'm working on a setup with two ST Link v2 programmers, so I have to select them using their serial numbers. Initially I was using the stm8flash binary from platformio. This worked fine. But after building from the most recent master, I got this error message:

$ stm8flash -S "523F6E06663F56493946103F" -c stlinkv2 -p "stm8s103?3" -s flash -w ki.hex
Determine FLASH area
ERROR: No programmer with serial number 523F6E06663F56493946103F found.
Couldn't initialize stlink

I added some debug output to check the comparison of the serial numbers:

fprintf(stdout, "a=\"%s\" b=\"%s\"\n", serialno_hex, pgm_serialno);

Which gave this output:

$ stm8flash -S 523F6E06663F56493946103F -c stlinkv2 -p "stm8s103?3" -s flash -w ki.hex
Determine FLASH area
a="523F6E06663F56493946103FX/��" b="523F6E06663F56493946103F"
a="373F6B064D56343143160843X/��" b="523F6E06663F56493946103F"
ERROR: No programmer with serial number 523F6E06663F56493946103F found.
Couldn't initialize stlink

Looking at the code, this makes sense. The string is never actually initialized with zeros. So I added the initialization, as well as a check to make sure at least the final zero byte is never overwritten.

This should fix the problem, at least as long as all programmer serials have the same length.

With these changes I can now successfully flash the target:

$ stm8flash -S 523F6E06663F56493946103F -c stlinkv2 -p "stm8s103?3" -s flash -w ki.hex
Determine FLASH area
a="523F6E06663F56493946103F" b="523F6E06663F56493946103F"
STLink: v2, JTAG: v29, SWIM: v7, VID: 8304, PID: 4837
Due to its file extension (or lack thereof), "ki.hex" is considered as INTEL HEX format!
8109 bytes at 0x8000... OK
Bytes written: 8109

Hope this helps :smiley_cat: