check_snmp: Fix regression introduced in #1867716

check_snmp: Fix regression introduced in #1867716

Bug #1867716 fixed what it meant to fix: broken perfdata strings.
Unfortunately some users relied on half-broken perfdata string where at
least the first token was OK. This patch do a two-way conversion
(string to double then back to string) instead and use the conversion
result for the performance data.

A possible caveat is that the string may change where it normally
shouldn't but the result should be somewhat similar.