vim-patch:8.2.4647: "source" can read past end of copied line

Problem:    "source" can read past end of copied line.
Solution:   Add a terminating NUL.

2bdad61267

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/runtime.c

@@ -2723,7 +2723,9 @@ static char *get_one_sourceline(source_cookie_T *sp)
       }
       ga_concat(&ga, ((char **)sp->buflines.ga_data)[sp->buf_lnum]);
       sp->buf_lnum++;
+      ga_grow(&ga, 1);
       buf = (char *)ga.ga_data;
+      buf[ga.ga_len++] = NUL;
     } else {
       buf = ga.ga_data;
 retry:

test/old/testdir/test_source.vim

@@ -638,4 +638,13 @@ func Test_source_buffer_vim9()
   %bw!
 endfunc
 
+func Test_source_buffer_long_line()
+  " This was reading past the end of the line.
+  new
+  norm300gr0
+  so
+  bwipe!
+endfunc
+
+
 " vim: shiftwidth=2 sts=2 expandtab