There was a incorrect else, that caused jq to not ensure that the
argument to strftime/1 is a string when the input is a number; this ends
up calling jv_string_value on a non-string value, which does not work,
and causes an assert failure.
Also fix same bug in strflocaltime/1.
Fixes https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=67403