clk: ti: check for null return in strrchr to avoid null dereferencing

strrchr can potentially return a null so the following strlen on the
null pointer can cause a null dereference. Add a check to see if
the string postfix is not null before calling strlen.

Detected by CoverityScan, CID#1452039 ("Dereference null return")

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Tero Kristo <t-kristo@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>