Fix error code in pk.h
diff --git a/library/pk.c b/library/pk.c
index a5f8d6b..ca99fc6 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -84,7 +84,7 @@
         return( 0 );
 
     if( ( ctx->data = malloc( size ) ) == NULL )
-        return( -1 );
+        return( POLARSSL_ERR_PK_MALLOC_FAILED );
 
     memset( ctx->data, 0, size );
     ctx->type = type;