Do not allocate dummy data

As reported by https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=69055

src/mat5.c

@@ -1643,7 +1643,7 @@ ReadNextStructField(mat_t *mat, matvar_t *matvar)
                         } else {
                             Mat_Critical("Couldn't determine file position");
                         }
-                        if ( fields[i]->internal->data != NULL ||
+                        if ( fields[i]->internal->data != NULL || nBytes <= (1 << MAX_WBITS) ||
                              fields[i]->class_type == MAT_C_STRUCT ||
                              fields[i]->class_type == MAT_C_CELL ) {
                             /* Memory optimization: Free inflate state */
@@ -3085,10 +3085,10 @@ Mat_VarRead5(mat_t *mat, matvar_t *matvar)
 
     if ( matvar == NULL )
         return MATIO_E_BAD_ARGUMENT;
-    else if ( matvar->rank == 0 ) /* An empty data set */
+    if ( matvar->rank == 0 || matvar->nbytes == 0 ) /* An empty data set */
         return MATIO_E_NO_ERROR;
 #if HAVE_ZLIB
-    else if ( NULL != matvar->internal->data ) {
+    if ( NULL != matvar->internal->data ) {
         /* Data already read in ReadNextStructField or ReadNextCell */
         matvar->data = matvar->internal->data;
         matvar->internal->data = NULL;
@@ -3229,18 +3229,14 @@ Mat_VarRead5(mat_t *mat, matvar_t *matvar)
                 break;
             }
             if ( 0 == matvar->nbytes ) {
-                matvar->data = calloc(1, 1);
-            } else {
-                matvar->data = calloc(matvar->nbytes, 1);
+                break;
             }
+            matvar->data = calloc(matvar->nbytes, 1);
             if ( NULL == matvar->data ) {
                 err = MATIO_E_OUT_OF_MEMORY;
                 Mat_Critical("Couldn't allocate memory for the data");
                 break;
             }
-            if ( 0 == matvar->nbytes ) {
-                break;
-            }
             {
                 size_t nbytes = 0;
                 err = Mul(&nbytes, nelems, matvar->data_size);