avutil/avsscanf: fix possible overreads when dealing with %c or %s

libavutil/avsscanf.c

@@ -113,7 +113,7 @@ static int ffshgetc(FFFILE *f)
 }
 
 #define shlim(f, lim) ffshlim((f), (lim))
-#define shgetc(f) (((f)->rpos != (f)->shend) ? *(f)->rpos++ : ffshgetc(f))
+#define shgetc(f) (((f)->rpos < (f)->shend) ? *(f)->rpos++ : ffshgetc(f))
 #define shunget(f) ((f)->shend ? (void)(f)->rpos-- : (void)0)
 
 static const unsigned char table[] = { -1,