abg-reader: clarify boolean use of assignment

abg-reader: clarify boolean use of assignment

When compiling with clang, the following warning is emitted:

abg-reader.cc:1981:15: warning: using the result of an assignment as a condition without parentheses [-Wparentheses]
  while (corp = read_corpus_from_input(ctxt))
         ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

That is certainly a common pitfall and can be mitigated by placing
parentheses around the assignment.

 * src/abg-reader.cc: clarify boolean use of assignment

Signed-off-by: Matthias Maennich <maennich@google.com>