LibMarkdown: Handle broken link markup better

Let's output *something* instead of crashing on a failed assertion.

Libraries/LibMarkdown/Text.cpp

@@ -24,6 +24,7 @@
  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 
+#include <AK/ScopeGuard.h>
 #include <AK/StringBuilder.h>
 #include <LibMarkdown/Text.h>
 #include <string.h>
@@ -158,12 +159,14 @@ bool Text::parse(const StringView& str)
  int first_span_in_the_current_link = -1;
 
  auto append_span_if_needed = [&](size_t offset) {
+ ASSERT(current_span_start <= offset);
  if (current_span_start != offset) {
  Span span {
  unescape(str.substring_view(current_span_start, offset - current_span_start)),
  current_style
  };
  m_spans.append(move(span));
+ current_span_start = offset;
  }
  };
 
@@ -199,31 +202,41 @@ bool Text::parse(const StringView& str)
  }
  break;
  case '[':
- ASSERT(first_span_in_the_current_link == -1);
+ if (first_span_in_the_current_link != -1)
+ dbg() << "Dropping the outer link";
  first_span_in_the_current_link = m_spans.size();
  break;
  case ']': {
- ASSERT(first_span_in_the_current_link != -1);
- ASSERT(offset + 2 < str.length());
- offset++;
- ASSERT(str[offset] == '(');
- offset++;
+ if (first_span_in_the_current_link == -1) {
+ dbg() << "Unmatched ]";
+ continue;
+ }
+ ScopeGuard guard = [&] {
+ first_span_in_the_current_link = -1;
+ };
+ if (offset + 2 >= str.length() || str[offset + 1] != '(')
+ continue;
+ offset += 2;
  size_t start_of_href = offset;
 
  do
  offset++;
  while (offset < str.length() && str[offset] != ')');
+ if (offset == str.length())
+ offset--;
 
  const StringView href = str.substring_view(start_of_href, offset - start_of_href);
  for (size_t i = first_span_in_the_current_link; i < m_spans.size(); i++)
  m_spans[i].style.href = href;
- first_span_in_the_current_link = -1;
  break;
  }
  default:
  ASSERT_NOT_REACHED();
  }
 
+ // We've processed the character as a special, so the next offset will
+ // start after it. Note that explicit continue statements skip over this
+ // line, effectively treating the character as not special.
  current_span_start = offset + 1;
  }