LibRegex: Prevent patterns from matching the empty string twice

Previously, if a pattern matched the empty string (e.g. ".*"), it would
match the string twice instead of once. Among other issues, this caused
a Regex replacement to duplicate its expected output, since it would
replace "both" empty matches.

Userland/Libraries/LibRegex/RegexMatcher.cpp

@@ -222,6 +222,11 @@ RegexResult Matcher<Parser>::match(Vector<RegexStringView> const& views, Optiona
                     // Nothing was *actually* matched, so append an empty match.
                     append_match(input, state, view_index);
                     ++match_count;
+
+                    // This prevents a regex pattern like ".*" from matching the empty string
+                    // multiple times, once in this block and once in the following for loop.
+                    if (view_index == 0 && view_length == 0)
+                        ++view_index;
                 }
             }
         }