Fix pnacl-sz to not accept files containing multiple modules.

Fix pnacl-sz to not accept files containing multiple modules.

Previous code checked, after the fact, that there weren't multiple
modules in the block. However, the code for TopLevelParser assumes
that there is only on module block, allowing it to more efficiently
test internal state. To fix the problem, modified
TopLevelParser::ParseBlock to fail, and not attempt to parse the second
block if the file contains more than one block.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4260
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1362643002 .