Fix null-pointer-dereference in yasm_expr_get_intnum (#244)

diff --git a/libyasm/expr.c b/libyasm/expr.c
index 5b0c418..09ae112 100644
--- a/libyasm/expr.c
+++ b/libyasm/expr.c
@@ -1264,7 +1264,7 @@
 {
     *ep = yasm_expr_simplify(*ep, calc_bc_dist);
 
-    if ((*ep)->op == YASM_EXPR_IDENT && (*ep)->terms[0].type == YASM_EXPR_INT)
+    if (*ep && (*ep)->op == YASM_EXPR_IDENT && (*ep)->terms[0].type == YASM_EXPR_INT)
         return (*ep)->terms[0].data.intn;
     else
         return (yasm_intnum *)NULL;