at91sam7: ensure probed flash bank has a name (fix a segfault)

at91sam7: ensure probed flash bank has a name (fix a segfault)

Before this commit, openocd used to segfault when probing flash
of an at91sam7x512 (which contains 2 banks of flash). This was due
to the way it systematically insert a new flash bank without setting
its name.
Then, when get_flash_bank_by_name_noprobe() is called, it is doing
a strcmp() on the non-initialized bank->name.

This commit prevents allocation of second probed bank if it is already
allocated (for example, if it is set in a target config file). If a
new bank really needs to be allocated, it ensures that a default name
is set.

Change-Id: I38d15bef1fda2ec746efad37171975136cf7b371
Signed-off-by: Aurelien Jacobs <aurel@gnuage.org>
Reviewed-on: http://openocd.zylin.com/171
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>