rctest: Fix compilation errors due to unaligned memory access

rctest: Fix compilation errors due to unaligned memory access

This fix following build errors on ARM.

  CC     test/rctest.o
test/rctest.c: In function do_send:
test/rctest.c:539:4: error: cast increases required alignment of target
	type [-Werror=cast-align]
test/rctest.c:540:4: error: cast increases required alignment of target
	type [-Werror=cast-align]
cc1: all warnings being treated as errors
make[1]: *** [test/rctest.o] Error 1