vim-patch:8.2.4646: using buffer line after it has been freed (#17907)

Problem:    Using buffer line after it has been freed in old regexp engine.
Solution:   After getting mark get the line again.
b55986c52d

src/nvim/regexp_bt.c

@@ -3131,8 +3131,16 @@ static bool regmatch(
           int mark = OPERAND(scan)[0];
           int cmp = OPERAND(scan)[1];
           pos_T *pos;
+          size_t col = REG_MULTI ? rex.input - rex.line : 0;
 
           pos = getmark_buf(rex.reg_buf, mark, false);
+
+          // Line may have been freed, get it again.
+          if (REG_MULTI) {
+            rex.line = reg_getline(rex.lnum);
+            rex.input = rex.line + col;
+          }
+
           if (pos == NULL                    // mark doesn't exist
               || pos->lnum <= 0) {           // mark isn't set in reg_buf
             status = RA_NOMATCH;

src/nvim/testdir/test_regexp_latin.vim

@@ -789,10 +789,17 @@ endfunc
 
 func Test_using_mark_position()
   " this was using freed memory
+  " new engine
   new
   norm O0
   call assert_fails("s/\\%')", 'E486:')
   bwipe!
+
+  " old engine
+  new
+  norm O0
+  call assert_fails("s/\\%#=1\\%')", 'E486:')
+  bwipe!
 endfunc
 
 func Test_using_visual_position()