brymen-dmm: improve text to number conversion robustness, signed OL

brymen-dmm: improve text to number conversion robustness, signed OL

The BM850(a/s) response packets are a mix of binary and text content.
The text part of it is _not_ a regular ASCIIZ string. Enforce the NUL
termination before running standard C library string routines on it.
Rephrase the conversion routine to become more C idiomatic.

Also check the optional sign for overflow conditions, return either
positive or negative infinity as a result.