malloc: size is unsigned, don't test for size < 0

clang complains

Change-Id: Ifadf73cf377c0d1808e20731803e01101bad7e1d
Signed-off-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-on: http://review.coreboot.org/780
Tested-by: build bot (Jenkins)
Reviewed-by: Mathias Krause <minipli@googlemail.com>