lib-sieve: plugins: include: ext-include-binary - Properly check whether binary dependency block load succeeded.

Otherwise, a segfault may occur on a corrupt binary.

The sieve_binary_block_index/get() functions return NULL when the block index is
invalid somehow. This was not handled by the dumper, thereby causing a crash.

Problem reported by scan-build.

lib-sieve: plugins: imap4flags: test-hasflag - Fix segfault occurring for command without arguments.

This is a corner case that doesn't happen in practice, but it can be created in
the test suite.

This reverts commit ffbcd3bf.

This applies to the vacation and notify actions, which produce new free-form
outgoing messages.

lib-sieve: enotify extension: mailto: Enforce subject header limit in Unicode codepoints rather than bytes.

It was "i;octet" previously, which is not standard.

This fixes a read overflow with Sieve environment extension accessing a
bare prefix, which might have ended up causing a crash. Currently, this
affected only attempting to access "vnd.dovecot.config" environment.