SF #1377897, Bus error in ast

If a line had multiple semi-colons and ended with a semi-colon, we would
loop too many times and access a NULL node.  Exit the loop early if
there are no more children.

Lib/test/test_grammar.py

@@ -276,6 +276,10 @@ check_syntax("lambda x: x = 2")
 ### simple_stmt: small_stmt (';' small_stmt)* [';']
 print 'simple_stmt'
 x = 1; pass; del x
+def foo():
+    # verify statments that end with semi-colons
+    x = 1; pass; del x;
+foo()
 
 ### small_stmt: expr_stmt | print_stmt  | pass_stmt | del_stmt | flow_stmt | import_stmt | global_stmt | access_stmt | exec_stmt
 # Tested below

Python/ast.c

@@ -2562,6 +2562,11 @@ ast_for_suite(struct compiling *c, const node *n)
 		ch = CHILD(ch, 0);
 		REQ(ch, simple_stmt);
 		for (j = 0; j < NCH(ch); j += 2) {
+		    /* statement terminates with a semi-colon ';' */
+		    if (NCH(CHILD(ch, j)) == 0) {
+		    	assert((j + 1) == NCH(ch));
+		    	break;
+		    }
 		    s = ast_for_stmt(c, CHILD(ch, j));
 		    if (!s)
 			goto error;