basic: remove an assertion from cunescape_one

The function takes a pointer to a random block of memory and
the length of that block. It shouldn't crash every time it sees
a zero byte at the beginning there.

This should help the dev-kmsg fuzzer to keep going.

src/basic/escape.c

@@ -106,7 +106,6 @@ int cunescape_one(const char *p, size_t length, char32_t *ret, bool *eight_bit)
         int r = 1;
 
         assert(p);
-        assert(*p);
         assert(ret);
 
         /* Unescapes C style. Returns the unescaped character in ret.