logger: add mutex to deferred logger (#391)

## Link to GitHub Issue, if one exists
[#390 ](https://github.com/spice2x/spice2x.github.io/pull/390)

## Description of change
Just like the normal logger, deferred logging can be called from any
component at any time, so accessing the vector needs to be guarded.

## Testing

src/spice2x/avs/core.cpp

@@ -1743,7 +1743,7 @@ namespace avs {
             deferredlogs::defer_error_messages({
                 "AVS filesystem initialization failure was previously detected during boot!",
                 fmt::format("    ERROR: directory could not be created: {}", src_path.string().c_str()),
-                fmt::format("    this crash may have been caused by bad <mounttable> contents in {}", avs::core::CFG_PATH.c_str()),
+                fmt::format("    if you see a crash, it may have been caused by bad <mounttable> contents in {}", avs::core::CFG_PATH.c_str()),
                 "    fix the XML file and try again"  
             });
         }

src/spice2x/util/deferlog.cpp

@@ -13,15 +13,18 @@ namespace deferredlogs {
         "    double check your spice audio options/patches, and try again"
     };
 
+    std::mutex deferred_errors_mutex;
     std::vector<std::vector<std::string>> deferred_errors;
 
     void defer_error_messages(std::initializer_list<std::string> messages) {
+        std::unique_lock<std::mutex> lock(deferred_errors_mutex);
         deferred_errors.emplace_back(messages);
     }
 
     void dump_to_logger() {
         static std::once_flag printed;
         std::call_once(printed, []() {
+            std::unique_lock<std::mutex> lock(deferred_errors_mutex);
             if (!deferred_errors.empty()) {
                 log_warning("deferred_error",
                     "!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!");