fix deadlock in rare cases when injecting an asset (#441)

docs/release-notes.md

@@ -7,6 +7,7 @@
 
 * For modders:
   * Fixed error when accessing a mod-provided API whose underlying class is `internal`.
+  * Fixed deadlock in rare cases when injecting a file with an asset loader.
 
 * For SMAPI developers:
   * All mod assemblies are now rewritten in-memory to support features like mod-provided APIs.

src/SMAPI/Framework/SContentManager.cs

@@ -792,12 +792,12 @@ namespace StardewModdingAPI.Framework
         {
             try
             {
-                this.Lock.EnterReadLock();
+                this.Lock.EnterWriteLock();
                 return action();
             }
             finally
             {
-                this.Lock.ExitReadLock();
+                this.Lock.ExitWriteLock();
             }
         }
     }