Remove erroneous abort() call

The call was introduced by commit ccef8581. It caused tar to abort
on perfectly normal operations, like untarring archives containing
./ with the -U option,

See http://lists.gnu.org/archive/html/bug-tar/2019-01/msg00019.html
for details.

* src/extract.c (maybe_recoverable): Remove misplaced call to abort().

src/extract.c

@@ -787,7 +787,7 @@ maybe_recoverable (char *file_name, bool regular, bool *interdir_made)
 	case UNLINK_FIRST_OLD_FILES:
 	  break;
 	}
-      abort (); /* notreached */
+      FALLTHROUGH;
 
     case ENOENT:
       /* Attempt creating missing intermediate directories.  */