gh-140594: Fix an out of bounds read when feeding NUL byte to PyOS_StdioReadline

Misc/NEWS.d/next/Security/2025-11-02-16-23-17.gh-issue-140594.YIWUpl.rst

@@ -0,0 +1,2 @@
+Fixed heap-buffer-overflow in PyOS_StdioReadline when encountering null
+bytes in interactive input. Patch by Shamil Abdulaev.

Parser/myreadline.c

@@ -344,7 +344,7 @@ PyOS_StdioReadline(FILE *sys_stdin, FILE *sys_stdout, const char *prompt)
             break;
         }
         n += strlen(p + n);
-    } while (p[n-1] != '\n');
+    } while (n > 0 && p[n-1] != '\n');
 
     pr = (char *)PyMem_RawRealloc(p, n+1);
     if (pr == NULL) {