mach-snapdragon: add a check before copying FDT to fdt_addr_r

mach-snapdragon: add a check before copying FDT to fdt_addr_r

The board_late_init() function allocates memory for a bunch of
environment variables, including fdt_addr_r. The device-tree then gets
copied to the memory pointed to by fdt_addr_r. However, the memory
allocation request can fail, in which case the address that is being
written to would not be allocated. Add a check that the memory
allocation has succeeded before copying the device-tree.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>