vim-patch:partial:9.0.0013: reproducing memory access errors can be difficult

Problem:    Reproducing memory access errors can be difficult.
Solution:   When testing, copy each line to allocated memory, so that valgrind
            can detect accessing memory before and/or after it.  Fix uncovered
            problems.

fa4873ccfc

Since test_override() is N/A, enable ml_get_alloc_lines when ASAN is
enabled instead, so it also applies to functional tests.

Use xstrdup() to copy the line as ml_line_len looks hard to port.

Squash the test changes from patch 9.0.0016.

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

src/nvim/memline_defs.h

@@ -49,10 +49,11 @@ typedef struct memline {
   int ml_stack_top;             // current top of ml_stack
   int ml_stack_size;            // total number of entries in ml_stack
 
-#define ML_EMPTY        1       // empty buffer
-#define ML_LINE_DIRTY   2       // cached line was changed and allocated
-#define ML_LOCKED_DIRTY 4       // ml_locked was changed
-#define ML_LOCKED_POS   8       // ml_locked needs positive block number
+#define ML_EMPTY        0x01    // empty buffer
+#define ML_LINE_DIRTY   0x02    // cached line was changed and allocated
+#define ML_LOCKED_DIRTY 0x04    // ml_locked was changed
+#define ML_LOCKED_POS   0x08    // ml_locked needs positive block number
+#define ML_ALLOCATED    0x10    // ml_line_ptr is an allocated copy
   int ml_flags;
 
   linenr_T ml_line_lnum;        // line number of cached line, 0 if not valid