Changed the thread_pool so it uses auto_mutex::unlock() in the appropriate places

instead of mutex::unlock().

dlib/threads/thread_pool_extension.cpp

@@ -275,7 +275,7 @@ namespace dlib
             // aren't any other worker threads free so just perform the task right
             // here
 
-            m.unlock();
+            M.unlock();
             bfp();
 
             // return a task id that is both non-zero and also one

dlib/threads/thread_pool_extension.h

@@ -182,7 +182,7 @@ namespace dlib
                 // aren't any other worker threads free so just perform the task right
                 // here
 
-                m.unlock();
+                M.unlock();
                 (obj.*funct)();
 
                 // return a task id that is both non-zero and also one
@@ -225,7 +225,7 @@ namespace dlib
                 // aren't any other worker threads free so just perform the task right
                 // here
 
-                m.unlock();
+                M.unlock();
                 (obj.*funct)(arg1);
 
                 // return a task id that is both non-zero and also one
@@ -270,7 +270,7 @@ namespace dlib
                 // aren't any other worker threads free so just perform the task right
                 // here
 
-                m.unlock();
+                M.unlock();
                 (obj.*funct)(arg1, arg2);
 
                 // return a task id that is both non-zero and also one