bpo-42150: Avoid buffer overflow in the new parser (GH-22978)

(cherry picked from commit e68c678)

Co-authored-by: Pablo Galindo <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2020-10-25-21-14-18.bpo-42150.b70u_T.rst

@@ -0,0 +1,2 @@
+Fix possible buffer overflow in the new parser when checking for
+continuation lines. Patch by Pablo Galindo.

Parser/pegen/pegen.c

@@ -989,7 +989,8 @@ bad_single_statement(Parser *p)
 
     /* Newlines are allowed if preceded by a line continuation character
        or if they appear inside a string. */
-    if (!cur || *(cur - 1) == '\\' || newline_in_string(p, cur)) {
+    if (!cur || (cur != p->tok->buf && *(cur - 1) == '\\')
+             || newline_in_string(p, cur)) {
         return 0;
     }
     char c = *cur;