maint: suppress a theoretical buffer overflow warning

maint: suppress a theoretical buffer overflow warning

* src/numfmt.c (double_to_human): The printf format is built up in
a stack buffer which is big enough to hold any of the possible formats.
However the size parameter passed to snprintf was too big by 1
when GROUP was true.  So decrease the buffer available to snprintf
to avoid this theoretical in practise but valid coverity warning.