fuzz: drop size limit again

The fuzzer keeps finding regexes that just fit into the limit, have a
Unicode word boundary assertion and gives a decent sized haystack. This
in turn results in slowish searches. The searches are horrificly slow,
but they become much slower with the sanitizers enabled it looks like.

So... drop the size limit down even more.

fuzz/fuzz_targets/fuzz_regex_match.rs

@@ -15,7 +15,7 @@ fuzz_target!(|data: &[u8]| {
         if let Some((char_index, _)) = char_index {
             let (pattern, input) = data.split_at(char_index);
             let result =
-                regex::RegexBuilder::new(pattern).size_limit(1 << 20).build();
+                regex::RegexBuilder::new(pattern).size_limit(1 << 18).build();
             if let Ok(re) = result {
                 re.is_match(input);
             }