[fuzz]added an input check in writefilter fuzzer and added test cases (…

…#12628)

Added a handle for nullptr in HeaderPercentageProvider::percentage to avoid crash in mongo_proxy.
Added many unit test cases into corpus so that the coverage can be improved. All those filters' coverage was increased by 20%-40%.
Signed-off-by: jianwen <[email protected]>

source/extensions/filters/common/fault/fault_config.cc

@@ -13,6 +13,10 @@ namespace Fault {
 
 envoy::type::v3::FractionalPercent
 HeaderPercentageProvider::percentage(const Http::RequestHeaderMap* request_headers) const {
+  if (request_headers == nullptr) {
+    // If request_headers is nullptr, return the default percentage.
+    return percentage_;
+  }
   const auto header = request_headers->get(header_name_);
   if (header == nullptr) {
     return percentage_;