Add fuzzers for consensus diff backend code

This takes two fuzzers: one which generates a diff and makes sure it
works, and one which applies a diff.

So far, they won't crash, but there's a bug in my
string-manipulation code someplace that I'm having to work around,
related to the case where you have a blank line at the end of a
file, or where you diff a file with itself.

src/test/fuzz/fuzz_diff.c

@@ -0,0 +1,67 @@
+/* Copyright (c) 2016, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+#define CONSDIFF_PRIVATE
+
+#include "orconfig.h"
+#include "or.h"
+#include "consdiff.h"
+
+#include "fuzzing.h"
+
+static int
+mock_consensus_compute_digest_(const char *c, consensus_digest_t *d)
+{
+  (void)c;
+  memset(d->sha3_256, 3, sizeof(d->sha3_256));
+  return 0;
+}
+
+int
+fuzz_init(void)
+{
+  MOCK(consensus_compute_digest, mock_consensus_compute_digest_);
+  return 0;
+}
+
+int
+fuzz_cleanup(void)
+{
+  UNMOCK(consensus_compute_digest);
+  return 0;
+}
+
+int
+fuzz_main(const uint8_t *stdin_buf, size_t data_size)
+{
+#define SEP "=====\n"
+#define SEPLEN strlen(SEP)
+  const uint8_t *separator = tor_memmem(stdin_buf, data_size, SEP, SEPLEN);
+  if (! separator)
+    return 0;
+  size_t c1_len = separator - stdin_buf;
+  char *c1 = tor_memdup_nulterm(stdin_buf, c1_len);
+  size_t c2_len = data_size - c1_len - SEPLEN;
+  char *c2 = tor_memdup_nulterm(separator + SEPLEN, c2_len);
+
+  char *c3 = consensus_diff_generate(c1, c2);
+
+  if (c3) {
+    char *c4 = consensus_diff_apply(c1, c3);
+    tor_assert(c4);
+    if (strcmp(c2, c4)) {
+      printf("%s\n", escaped(c1));
+      printf("%s\n", escaped(c2));
+      printf("%s\n", escaped(c3));
+      printf("%s\n", escaped(c4));
+    }
+    tor_assert(! strcmp(c2, c4));
+    tor_free(c3);
+    tor_free(c4);
+  }
+  tor_free(c1);
+  tor_free(c2);
+
+  return 0;
+}
+