nimble/host: Fix invalid memset in ble_gattc_read_mult_cb_var

This commit applies correct value when using memset to fill attr
array. Fixes GATT/CL/GAR/BI-13-C, GATT/CL/GAR/BI-36-C,
GATT/CL/GAR/BI-38-C, GATT/CL/GAR/BI-42-C, GATT/CL/GAR/BI-44-C,
GATT/CL/GAR/BV-05-C

nimble/host/src/ble_gattc.c

@@ -3345,7 +3345,7 @@ ble_gattc_read_mult_cb_var(struct ble_gattc_proc *proc, int status,
         return 0;
     }
 
-    memset(attr, 0, sizeof(*attr));
+    memset(attr, 0, sizeof(*attr) * proc->read_mult.num_handles);
 
     for (i = 0; i < proc->read_mult.num_handles; i++) {
         attr[i].handle = proc->read_mult.handles[i];