logging/Ascii: Fix abort() for non-existing postrotation functions

When looking up the postprocessor function from shadow files, id::find_func()
would abort() if the function wasn't available instead of falling back
to the default postprocessor.

Fix by using id::find() and checking the type explicitly and also adding a
strict type check while at it.

This issue was tickled by loading the json-streaming-logs package,
Zeek creating shadow files containing its custom postprocessor function,
then restarting Zeek without the package loaded.

Closes #4562

testing/btest/scripts/base/frameworks/logging/rotate-leftover-log-rotation-shadow-files-postprocessor-errors.zeek

@@ -0,0 +1,32 @@
+# @TEST-DOC: Put a non-existing postprocessor function into a shadow file, ensure the default gets picked up. Regression test for #4562
+#
+# @TEST-EXEC: echo ".log" >> .shadow.conn.log
+# @TEST-EXEC: echo "non_existing_rotation_postprocessor" >> .shadow.conn.log
+# @TEST-EXEC: echo "leftover conn log" > conn.log
+#
+# @TEST-EXEC: echo ".log" >> .shadow.dns.log
+# @TEST-EXEC: echo "wrongly_typed_rotation_postprocessor" >> .shadow.dns.log
+# @TEST-EXEC: echo "leftover dns log" > dns.log
+#
+# @TEST-EXEC: zeek -b %INPUT
+# @TEST-EXEC: TEST_DIFF_CANONIFIER="$SCRIPTS/diff-remove-abspath | $SCRIPTS/diff-sort" btest-diff .stderr
+#
+# Ensure leftover files were removed.
+# @TEST-EXEC: ! test -f .shadow.conn.log
+# @TEST-EXEC: ! test -f conn.log
+# @TEST-EXEC: ! test -f .shadow.dns.log
+# @TEST-EXEC: ! test -f dns.log
+#
+# Ensure the rotated conn log ends-up in the current working directory.
+# @TEST-EXEC: cat ./conn.*.log ./dns.*.log > logs.cat
+# @TEST-EXEC: btest-diff logs.cat
+
+function wrongly_typed_rotation_postprocessor(): bool
+	{
+	exit(1);
+	return T;
+	}
+
+redef LogAscii::enable_leftover_log_rotation = T;
+redef Log::default_rotation_interval = 1hr;
+redef Log::default_rotation_date_format = "no-date";