ossfuzz: Add simple mruby compile test harness

oss-fuzz/mruby_fuzzer.c

+#include <stdlib.h>
+#include <string.h>
+#include <mruby.h>
+#include <mruby/compile.h>
+
+int LLVMFuzzerTestOneInput(uint8_t *Data, size_t size) {
+    if (size < 1) {
+        return 0;
+    }
+    char *code = malloc(size+1);
+    memcpy(code, Data, size);
+    code[size] = '\0';
+    mrb_state *mrb = mrb_open();
+    mrb_load_string(mrb, code);
+    mrb_close(mrb);
+    free(code);
+    return 0;
+}