of: properly check for error returned by fdt_get_name()

fdt_get_name() returns error values via a parameter pointer
instead of in function return.  Fix check for this error value
in populate_node() and callers of populate_node().

Chasing up the caller tree showed callers of various functions
failing to initialize the value of pointer parameters that
can return error values.  Initialize those values to NULL.

The bug was introduced by
commit e6a6928c ("of/fdt: Convert FDT functions to use libfdt")
but this patch can not be backported directly to that commit
because the relevant code has further been restructured by
commit dfbd4c6e ("drivers/of: Split unflatten_dt_node()")

The bug became visible by triggering a crash on openrisc with:
commit 79edff12 ("scripts/dtc: Update to upstream version v1.6.0-51-g183df9e9c2b9")
as reported in:
https://lore.kernel.org/lkml/20210327224116.69309-1-linux@roeck-us.net/

Fixes: 79edff12

 ("scripts/dtc: Update to upstream version v1.6.0-51-g183df9e9c2b9")
Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Frank Rowand <frank.rowand@sony.com>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20210405032845.1942533-1-frowand.list@gmail.com
Signed-off-by: Rob Herring <robh@kernel.org>