[gcc r13-7916] libstdc++: fix illegal pointer arithmetic in format [PR111102]

[Jonathan Wakely]

https://gcc.gnu.org/g:183eea6029be2f6c9f416d6ffe751c469237ff2d

commit r13-7916-g183eea6029be2f6c9f416d6ffe751c469237ff2d
Author: Paul Dreik <gccpatches@pauldreik.se>
Date:   Thu Aug 24 11:43:43 2023 +0100

    libstdc++: fix illegal pointer arithmetic in format [PR111102]
    
    When parsing a format string, the width is parsed into an unsigned short
    but the result is not checked in the case the format string is not a
    char string (such as a wide string). In case the parse fails, a null
    pointer is returned which is used for pointer arithmetic which is
    undefined behaviour.
    
    Signed-off-by: Paul Dreik <gccpatches@pauldreik.se>
    
    libstdc++-v3/ChangeLog:
    
            PR libstdc++/111102
            * include/std/format (__format::__parse_integer): Check for
            non-null pointer.
    
    (cherry picked from commit dd4bdb9eea436bf06f175d8dbfc2190377455be4)

Diff:
---
 libstdc++-v3/include/std/format | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/std/format b/libstdc++-v3/include/std/format
index ec910f13a8e..3721f021afd 100644
--- a/libstdc++-v3/include/std/format
+++ b/libstdc++-v3/include/std/format
@@ -285,7 +285,8 @@ namespace __format
 	  for (int __i = 0; __i < __n && (__first + __i) != __last; ++__i)
 	    __buf[__i] = __first[__i];
 	  auto [__v, __ptr] = __format::__parse_integer(__buf, __buf + __n);
-	  return {__v, __first + (__ptr - __buf)};
+	  if (__ptr) [[likely]]
+	    return {__v, __first + (__ptr - __buf)};
 	}
       return {0, nullptr};
     }