RSA PSS: remove redundant check; changelog

Remove a check introduced in the previous buffer overflow fix with keys of
size 8N+1 which the subsequent fix for buffer start calculations made
redundant.

Added a changelog entry for the buffer start calculation fix.
diff --git a/library/rsa.c b/library/rsa.c
index b54960f..148f6b3 100644
--- a/library/rsa.c
+++ b/library/rsa.c
@@ -1399,8 +1399,7 @@
     while( p < hash_start - 1 && *p == 0 )
         p++;
 
-    if( p == hash_start ||
-        *p++ != 0x01 )
+    if( *p++ != 0x01 )
     {
         mbedtls_md_free( &md_ctx );
         return( MBEDTLS_ERR_RSA_INVALID_PADDING );