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.

src/scriptfile.c

@@ -1918,7 +1918,10 @@ get_one_sourceline(source_cookie_T *sp)
 		break;		    // all the lines are processed
 	    ga_concat(&ga, ((char_u **)sp->buflines.ga_data)[sp->buf_lnum]);
 	    sp->buf_lnum++;
+	    if (ga_grow(&ga, 1) == FAIL)
+		break;
 	    buf = (char_u *)ga.ga_data;
+	    buf[ga.ga_len++] = NUL;
 	}
 	else
 	{

src/testdir/test_source.vim

@@ -646,4 +646,13 @@ 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

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4647,
 /**/
     4646,
 /**/