journal: fix against (theoretical) undefined behavior

While all the libc implementations I know return NULL when memchr's size
parameter is 0, without accessing any memory, passing NULL to memchr is
still invalid:

C11 7.24.1p2: Where an argument declared as "size_t n" specifies the length
of the array for a function, n can have the value zero on a call to that
function. Unless explicitly stated otherwise in the description of a
particular function in this subclause, pointer arguments on such a call
shall still have valid values, as described in 7.1.4. On such a call, a
function that locates a character finds no occurrence, a function that
compares two character sequences returns zero, and a function that copies
characters copies zero characters.

see http://llvm.org/bugs/show_bug.cgi?id=18247

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