cdecode: avoid off-by-one read of decoding table.

If value_in is equal to decoding_size, we will overrun the decoding table
by one. Fix this by returning an error if the value is outside of the range
of the table.
Caught by Coverity.
armor: fix potential memory leak of pgp_plain.

If realloc() fails, it will return NULL, which we promptly wrote to the
pgp_plain pointer, upon trying to free() it afterwards. Not a problem in
and of itself, the problem being that realloc() does not free() the
original pointer if it fails, so this was a memory leak.
Caught by clang-analyze.
test-verify: close TOCTOU hole for verifying data.

Use open() and the resulting FD for all subsequent operations instead
of relying on "files/vmImage" being the same across multiple accesses.
Caught by Coverity.
Merge branch 'master' of /home/git/repositories/root/x-18789-libsign