rnd: Change parameter type in randombytes_{weak,strong}

Change parameter type for the length to size_t since this is what is
passed to them and is expected by read_exact().

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
rnd: Fix compiler warning

Fix the following compiler warning that occurs when building with "-W
-Wall -Wextra":

rnd.c: In function ‘randombytes_strong’:
rnd.c:50:10: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>