mprintf: use snprintf if available

mprintf: use snprintf if available

This is the single place in libcurl code where it uses the "native"
s(n)printf() function. Used for writing floats. The use has been
reviewed and vetted and uses a HUGE target buffer, but switching to
snprintf() still makes this safer and removes build-time warnings.

Reported-by: Philip Heiduck
Fixes #9569
Closes #9570