jplist: Improve numerical value parsing without copying data to stack buffer
jplist: Improve numerical value parsing without copying data to stack buffer

Instead of calling strtoll() and atof(), the code now parses the numerical
values directly to handle cases of non-0-terminated string data.
The floating point value parsing is probably not ideal, but sufficient for
our purposes.

libimobiledevice/libplist,libimobiledevice/libplist,libimobiledevice/libplist