mtdparts: fixed buffer overflow bug

In the case that there was no name defined for a partition the
code assumes that name_len is 22 and therefore allocates exactly
that space for a dummy name. But the function sprintf() first
resolves "0x%08llx@0x%08llx" to a string that is longer than 22
bytes. This leads to a buffer overflow. The replacement function
snprintf() limits the copied bytes to name_len and therefore
avoids the buffer overflow.

Signed-off-by: Kay Potthoff <Kay.Potthoff@microsys.de>