scan-build: memarea_strndup() undefined behavior

The memarea_strndup() function would have hit undefined behavior by
creating an 'end' pointer off the end of a string if it had ever been
given an 'n' argument bigger than the length of the memory ares that
it's scanning.  Fortunately, we never did that except in the unit
tests.  But it's not a safe behavior to leave lying around.

src/common/memarea.c

@@ -291,14 +291,11 @@ memarea_strdup(memarea_t *area, const char *s)
 char *
 memarea_strndup(memarea_t *area, const char *s, size_t n)
 {
-  size_t ln;
+  size_t ln = 0;
   char *result;
-  const char *cp, *end = s+n;
   tor_assert(n < SIZE_T_CEILING);
-  for (cp = s; cp < end && *cp; ++cp)
+  for (ln = 0; ln < n && s[ln]; ++ln)
     ;
-  /* cp now points to s+n, or to the 0 in the string. */
-  ln = cp-s;
   result = memarea_alloc(area, ln+1);
   memcpy(result, s, ln);
   result[ln]='\0';