Rectify parameters passed to strsep to avoid error EINVAL

[nil4git]

Function strcspn & strsep expects a null terminated string delimiter in this case so as to avoid corruption
This issue is observed with example incoming string B0L0000000C passed to rpmsg_sdb_decode_rxbuf_string
With delimiter set to {'L'} and not null terminated, whereas the function strcspn expects a null terminated string
This will result in delimiter with garbage value getting passed eventually resulting in EINVAL issue observed specifically in case of B0 as observed below
incoming string [B0L0000000C] rxbuf_str string [L0000000C] and subString is [B]

[nil4git]

@jctrotin @mcatrouillet @mcatrouST @timblechmann Kindly review
Thanks

This should close the open issue #2 as well