[iphone] Add missing va_start()/va_end() around reused argument list

[iphone] Add missing va_start()/va_end() around reused argument list

The ipair_tx() function uses a va_list twice (first to calculate the
formatted string length before allocation, then to construct the
string in the allocated buffer) but is missing the va_start() and
va_end() around the second usage.  This is undefined behaviour that
happens to work on some build platforms.

Fix by adding the missing va_start() and va_end() around the second
usage of the variadic argument list.

Reported-by: Andreas Hammarskjöld <andreas@2PintSoftware.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>