gdb_server: fix double free

gdb_server: fix double free

Commit 6541233aa78d ("Combine register lists of smp targets.")
unconditionally assigns the output pointers of the function
smp_reg_list_noread(), even if the function fails and returns
error.
This causes a double free from the caller, that has assigned NULL
to the pointers to simplify the error handling.

Use local variables in smp_reg_list_noread() and assign the output
pointers only on success.

Change-Id: Ic0fd2f26520566cf322f0190780e15637c01cfae
Fixes: 6541233aa78d ("Combine register lists of smp targets.")
Reported-by: Michele Bisogno <michele.bisogno.ct@renesas.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6852
Tested-by: jenkins
Reviewed-by: Michele Bisogno <michele.bisogno.ct@renesas.com>
Reviewed-by: Tim Newsome <tim@sifive.com>