Based on https://gmplib.org/repo/gmp-6.2/raw-rev/561a9c25298e,
adapted for 5.x by ris

diff -r e1fd9db13b47 -r 561a9c25298e mpz/inp_raw.c
--- a/mpz/inp_raw.c	Tue Dec 22 23:49:51 2020 +0100
+++ b/mpz/inp_raw.c	Thu Oct 21 19:06:49 2021 +0200
@@ -81,8 +81,11 @@
 
   abs_csize = ABS (csize);
 
+  if (UNLIKELY (abs_csize > ~(mp_bitcnt_t) 0 / 8))
+    return 0; /* Bit size overflows */
+
   /* round up to a multiple of limbs */
-  abs_xsize = (abs_csize*8 + GMP_NUMB_BITS-1) / GMP_NUMB_BITS;
+  abs_xsize = ((mp_bitcnt_t)abs_csize*8 + GMP_NUMB_BITS-1) / GMP_NUMB_BITS;
 
   if (abs_xsize != 0)
     {