vim-patch:8.2.4974: ":so" command may read after end of buffer

Problem:    ":so" command may read after end of buffer.
Solution:   Compute length of text properly.

4748c4bd64

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

src/nvim/runtime.c

@@ -2628,6 +2628,7 @@ static char *get_one_sourceline(source_cookie_T *sp)
       ga_grow(&ga, 1);
       buf = (char *)ga.ga_data;
       buf[ga.ga_len++] = NUL;
+      len = ga.ga_len;
     } else {
       buf = ga.ga_data;
 retry:
@@ -2638,8 +2639,8 @@ retry:
         }
         break;
       }
+      len = ga.ga_len + (int)strlen(buf + ga.ga_len);
     }
-    len = ga.ga_len + (int)strlen(buf + ga.ga_len);
 #ifdef USE_CRNL
     // Ignore a trailing CTRL-Z, when in Dos mode. Only recognize the
     // CTRL-Z by its own, or after a NL.

test/old/testdir/test_source.vim

@@ -612,6 +612,17 @@ func Test_source_buffer_long_line()
   norm300gr0
   so
   bwipe!
+
+  let lines =<< trim END
+      new
+      norm 10a0000000000ø00000000000
+      norm i0000000000000000000
+      silent! so
+  END
+  call writefile(lines, 'Xtest.vim')
+  source Xtest.vim
+  bwipe!
+  call delete('Xtest.vim')
 endfunc