spapr: fix out of bounds write in spapr_populate_drmem_v2

buf_len is uint8_t which is not large enough to hold the result of:

  nr_entries * sizeof(struct sPAPRDrconfCellV2) + sizeof(uint32_t);

for a nr_entries greater than 10.

This causes the allocated buffer 'int_buf' to be smaller than expected
and we eventually overwrite some of glibc's control structures (see
"chunk" in https://sourceware.org/glibc/wiki/MallocInternals)

The following error is seen while trying to free int_buf:

  "free(): invalid next size (fast)"

Fixes: a324d6f1 "spapr: Support ibm,dynamic-memory-v2 property"
Signed-off-by: NFabiano Rosas <farosas@linux.ibm.com>
Message-Id: <20190213172926.21740-1-farosas@linux.ibm.com>
Reviewed-by: NGreg Kurz <groug@kaod.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/ppc/spapr.c

@@ -687,14 +687,14 @@ static int spapr_populate_drmem_v2(sPAPRMachineState *spapr, void *fdt,
                                    int offset, MemoryDeviceInfoList *dimms)
 {
     MachineState *machine = MACHINE(spapr);
-    uint8_t *int_buf, *cur_index, buf_len;
+    uint8_t *int_buf, *cur_index;
     int ret;
     uint64_t lmb_size = SPAPR_MEMORY_BLOCK_SIZE;
     uint64_t addr, cur_addr, size;
     uint32_t nr_boot_lmbs = (machine->device_memory->base / lmb_size);
     uint64_t mem_end = machine->device_memory->base +
                        memory_region_size(&machine->device_memory->mr);
-    uint32_t node, nr_entries = 0;
+    uint32_t node, buf_len, nr_entries = 0;
     sPAPRDRConnector *drc;
     DrconfCellQueue *elem, *next;
     MemoryDeviceInfoList *info;