drivers: dfu: ram: fix a crash with dfu ram with invalid dfu_alt_info env

U-Boot crashes when an invalid dfu_alt_info is set and tried
using dfu command. Fixing this as it is handled in dfu-mmc.

=> dfu 0 ram 0
data abort
pc : [<9ff893d6>]          lr : [<9ff6edb9>]
reloc pc : [<808323d6>]    lr : [<80817db9>]
sp : 9ef36cf0  ip : 00000158     fp : 9ffbc0b8
r10: 9ffbc0b8  r9 : 9ef36ed8     r8 : 00000000
r7 : 00000000  r6 : 9ffbc0c8     r5 : 9ef36cfc  r4 : 9ef392c8
r3 : 00000004  r2 : 00000000     r1 : 9ff9a985  r0 : ffffffff
Flags: Nzcv  IRQs off  FIQs on  Mode SVC_32
Resetting CPU ...

resetting ...

Signed-off-by: Mugunthan V N <mugunthanvnm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>