[PATCH 12/14] fix potential dereference of NULL in is_cyclic_region()

Found with -fanalyzer

In file included from sadump_info.c:21:
makedumpfile.h: In function ‘is_dumpable_buffer’:
makedumpfile.h:2314:24: warning: dereference of NULL ‘cycle’ [CWE-476] [-Wanalyzer-null-dereference]
 2314 |         if (pfn < cycle->start_pfn || cycle->end_pfn <= pfn)
      |                   ~~~~~^~~~~~~~~~~
...
  ‘sadump_initialize_bitmap_memory’: events 12-14
    |
    |  891 |                      pfn < (section + 1) * SADUMP_PF_SECTION_NUM;
    |  892 |                      ++pfn)
    |  893 |                         if (is_dumpable(bmp, pfn, NULL))
    |      |                         ~~  ~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |                         |   |
    |      |                         |   (14) calling ‘is_dumpable’ from ‘sadump_initialize_bitmap_memory’
    |      |                         (13) ...to here
    |
    +--> ‘is_dumpable’: events 15-18
           |
           |makedumpfile.h:2355:1:
           | 2355 | is_dumpable(struct dump_bitmap *bitmap, mdf_pfn_t pfn, struct cycle *cycle)
           |      | ^~~~~~~~~~~
           |      | |
           |      | (15) entry to ‘is_dumpable’
           | 2356 | {
           | 2357 |         if (bitmap->fd < 0) {
           |      |            ~
           |      |            |
           |      |            (16) following ‘true’ branch...
           | 2358 |                 return is_dumpable_buffer(bitmap, pfn, cycle);
           |      |                 ~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
           |      |                 |      |
           |      |                 |      (18) calling ‘is_dumpable_buffer’ from ‘is_dumpable’
           |      |                 (17) ...to here
           |
           +--> ‘is_dumpable_buffer’: events 19-20
                  |
                  | 2314 |         if (pfn < cycle->start_pfn || cycle->end_pfn <= pfn)
                  |      |                   ~~~~~~~~~~~~~~~~
                  |      |                        |
                  |      |                        (20) dereference of NULL ‘cycle’
                  |......
                  | 2321 | is_dumpable_buffer(struct dump_bitmap *bitmap, mdf_pfn_t pfn, struct cycle *cycle)
                  |      | ^~~~~~~~~~~~~~~~~~
                  |      | |
                  |      | (19) entry to ‘is_dumpable_buffer’
                  |

Signed-off-by: Philipp Rudo <prudo@redhat.com>