Fixed uninitialized memory issue in link table allocation. (#5936)

When H5G__dense_iterate failed early, the allocated H5O_link_t structures were never populated, causing cleanup code to read uninitialized lnk->type values and attempt to free invalid pointers.

The structures are now initialized with proper values and NULL pointers immediately after allocation.

Fixes GitHub #5375

Author: bmribler

src/H5Gdense.c

@@ -762,9 +762,13 @@ H5G__dense_build_table(H5F_t *f, const H5O_linfo_t *linfo, H5_index_t idx_type,
         H5G_dense_bt_ud_t udata; /* User data for iteration callback */
 
         /* Allocate the table to store the links */
-        if ((ltable->lnks = (H5O_link_t *)H5MM_malloc(sizeof(H5O_link_t) * ltable->nlinks)) == NULL)
+        if ((ltable->lnks = (H5O_link_t *)H5MM_calloc(sizeof(H5O_link_t) * ltable->nlinks)) == NULL)
             HGOTO_ERROR(H5E_RESOURCE, H5E_NOSPACE, FAIL, "memory allocation failed");
 
+        /* Initialize all links to invalid. NOTE: If H5O_link_t changes, update this loop. */
+        for (size_t i = 0; i < ltable->nlinks; i++)
+            ltable->lnks[i].type = H5L_TYPE_ERROR;
+
         /* Set up user data for iteration */
         udata.ltable   = ltable;
         udata.curr_lnk = 0;