Add missing lock releases in __register_exitproc().

In some code paths the __atexit_lock held by this function was not
released when returning with an error.

        * libc/stdlib/__atexit.c (__register_exitproc): Always release
        lock before return.

newlib/ChangeLog

@@ -1,3 +1,8 @@
+2015-12-12  Freddie Chopin  <freddie.chopin@gmail.com>
+
+	* libc/stdlib/__atexit.c (__register_exitproc): Always release
+	lock before return.
+
 2015-12-09  Thomas Preud'homme  <thomas.preudhomme@arm.com>
 
 	* libm/machine/arm/s_ceil.c: Also check that 64bit FP instructions are

newlib/libc/stdlib/__atexit.c

@@ -83,12 +83,20 @@ _DEFUN (__register_exitproc,
   if (p->_ind >= _ATEXIT_SIZE)
     {
 #ifndef _ATEXIT_DYNAMIC_ALLOC
+#ifndef __SINGLE_THREAD__
+      __lock_release_recursive(__atexit_lock);
+#endif
       return -1;
 #else
       /* Don't dynamically allocate the atexit array if malloc is not
 	 available.  */
       if (!malloc)
-	return -1;
+	{
+#ifndef __SINGLE_THREAD__
+	  __lock_release_recursive(__atexit_lock);
+#endif
+	  return -1;
+	}
 
       p = (struct _atexit *) malloc (sizeof *p);
       if (p == NULL)