time: fix warning in timestring creation

time: fix warning in timestring creation

cppcheck:

src/util-time.c:255:18: warning: Either the condition 'str!=NULL' is redundant or there is possible null pointer dereference: str. [nullPointerRedundantCheck]
        snprintf(str, size, "ts-error");
                 ^
src/util-time.c:252:48: note: Assuming that condition 'str!=NULL' is not redundant
    if (likely(t != NULL && fmt != NULL && str != NULL)) {
                                               ^
src/util-time.c:255:18: note: Null pointer dereference
        snprintf(str, size, "ts-error");
                 ^

Only `t` could possibly be NULL if `localtime_r` fails elsewhere.

Bug: #5291.