patch 9.0.0620: matchaddpos() can only add up to 8 matches

Problem:    matchaddpos() can only add up to 8 matches.
Solution:   Allocate the array of positions. (closes #11248)
diff --git a/src/testdir/test_match.vim b/src/testdir/test_match.vim
index f654761..91e78a7 100644
--- a/src/testdir/test_match.vim
+++ b/src/testdir/test_match.vim
@@ -219,6 +219,21 @@
   set hlsearch&
 endfunc
 
+" Add 12 match positions (previously the limit was 8 positions).
+func Test_matchaddpos_dump()
+  CheckScreendump
+
+  let lines =<< trim END
+      call setline(1, ['1234567890123']->repeat(14))
+      call matchaddpos('Search', range(1, 12)->map({i, v -> [v, v]}))
+  END
+  call writefile(lines, 'Xmatchaddpos', 'D')
+  let buf = RunVimInTerminal('-S Xmatchaddpos', #{rows: 14})
+  call VerifyScreenDump(buf, 'Test_matchaddpos_1', {})
+
+  call StopVimInTerminal(buf)
+endfunc
+
 func Test_matchaddpos_otherwin()
   syntax on
   new