Fixing #18659

Changes the radix multiplier to start at -0.0 instead of -1.0
when reading strings that start with '-'.
auto merge of #18713 : juxiliary/rust/master, r=alexcrichton

* `from_str_radix_float` gives incorrect results for negative float strings. Changes the accumulator used to start at -0.0 instead of -1.0.
* Adds missing tests