Enforce maximum length of fuzz input

Remove the libfuzzer max_len option which doesn't apply to other
fuzzing engines. Enforce the maximum length directly in the fuzz
targets. For the xml target, lower the maximum when expanding entities
to avoid timeout and OOM errors.

fuzz/xml.c

@@ -29,13 +29,18 @@ LLVMFuzzerTestOneInput(const char *data, size_t size) {
     xmlTextReaderPtr reader;
     xmlChar *out;
     const char *docBuffer, *docUrl;
-    size_t docSize, consumed, chunkSize;
+    size_t maxSize, docSize, consumed, chunkSize;
     int opts, outSize;
 
     xmlFuzzDataInit(data, size);
     opts = xmlFuzzReadInt();
-    /* XML_PARSE_HUGE still causes timeouts. */
-    opts &= ~XML_PARSE_HUGE;
+
+    /* Lower maximum size when processing entities for now. */
+    maxSize = opts & XML_PARSE_NOENT ? 50000 : 500000;
+    if (size > maxSize) {
+        xmlFuzzDataCleanup();
+        return(0);
+    }
 
     xmlFuzzReadEntities();
     docBuffer = xmlFuzzMainEntity(&docSize);