Fix problem with special chars in descriptions

Fix 'task add a; task add "\n"; task next' returning "Unknown error."
The problem can be reproduced with any report including "description"
as a column.

For task descriptions that had more special characters than regular
ones, "length" in utf8_width() underflowed, leading to problems
elsewhere in the code.

src/utf8.cpp

@@ -188,10 +188,21 @@ unsigned int utf8_length (const std::string& str)
 unsigned int utf8_width (const std::string& str)
 {
   unsigned int length = 0;
+  int l;
   std::string::size_type i = 0;
   unsigned int c;
   while ((c = utf8_next_char (str, i)))
-    length += mk_wcwidth (c);
+  {
+    l = mk_wcwidth (c);
+    // Control characters, and more especially newline characters, make
+    // mk_wcwidth() return -1.  Ignore that, thereby "adding zero" to length.
+    // Since control characters are not displayed in reports, this is a valid
+    // choice.
+    if (l != -1)
+    {
+      length += l;
+    }
+  }
 
   return length;
 }