env: sf: report malloc error to caller

In the non-redundant code for env_sf_save(), a failure to malloc() the
temporary buffer produces the following output:

    Saving Environment to SPIFlash... OK

This is misleading as the flash has neither been erased nor written.

Fix it to return an error to the caller, so the output will be:

    Saving Environment to SPIFlash... Failed (-12)

Note that there is another copy of env_sf_save() in the same file, for
handling redundant environment, and it already has the same logic.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>