[FL-2811] Fix PVS-Studio warnings (#2142)

Co-authored-by: あく <alleteam@gmail.com>
Co-authored-by: gornekich <n.gorbadey@gmail.com>

furi/core/mutex.c

@@ -30,6 +30,8 @@ FuriMutex* furi_mutex_alloc(FuriMutexType type) {
 
 void furi_mutex_free(FuriMutex* instance) {
     furi_assert(!FURI_IS_IRQ_MODE());
+    furi_assert(instance);
+
     vSemaphoreDelete((SemaphoreHandle_t)((uint32_t)instance & ~1U));
 }