Add a fuzzing harness for Puzzles

This just feeds save files into the loading code, but because of how
Puzzles is structured that actually exercises most of its parsers.

fuzzpuzz.c

@@ -0,0 +1,65 @@
+/*
+ * fuzzpuzz.c: Fuzzing frontend to all puzzles.
+ */
+
+/*
+ * The idea here is that this front-end supports all back-ends and can
+ * feed them save files.  This tests the deserialiser, the code for
+ * loading game descriptions, and the processing of move strings,
+ * without all the tedium of actually rendering anything.
+ */
+
+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "puzzles.h"
+
+static bool savefile_read(void *wctx, void *buf, int len)
+{
+    FILE *fp = (FILE *)wctx;
+    int ret;
+
+    ret = fread(buf, 1, len, fp);
+    return (ret == len);
+}
+
+int main(int argc, char **argv)
+{
+    const char *err;
+    char *gamename;
+    int i;
+    const game * ourgame = NULL;
+    midend *me;
+
+    if (argc != 1) {
+        fprintf(stderr, "usage: %s\n", argv[0]);
+        exit(1);
+    }
+
+    err = identify_game(&gamename, savefile_read, stdin);
+    if (err != NULL) {
+        fprintf(stderr, "%s\n", err);
+        exit(1);
+    }
+
+    for (i = 0; i < gamecount; i++)
+        if (strcmp(gamename, gamelist[i]->name) == 0)
+            ourgame = gamelist[i];
+    if (ourgame == NULL) {
+        fprintf(stderr, "Game '%s' not recognised\n", gamename);
+        exit(1);
+    }
+
+    me = midend_new(NULL, ourgame, NULL, NULL);
+
+    rewind(stdin);
+    err = midend_deserialise(me, savefile_read, stdin);
+    if (err != NULL) {
+        fprintf(stderr, "%s\n", err);
+        exit(1);
+    }
+    midend_free(me);
+    return 0;
+}