The bitcode parser assumes that the module symbol table, if it exists,
must appear before the first function
block (i.e. defininition). However, the parser did not check this
constraint. This patch fixes that omission.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4320
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1359923002 .