Fix overflow in c2i_ASN1_BIT_STRING.

c2i_ASN1_BIT_STRING takes length as a long but uses it as an int.  Check
bounds before doing so. Previously, excessively large inputs to the
function could write a single byte outside the target buffer. (This is
unreachable as asn1_ex_c2i already uses int for the length.)

Thanks to NCC for finding this issue. Fix written by Martin Kreichgauer.
Reviewed-by: NRichard Levitte <levitte@openssl.org>
Reviewed-by: NAndy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/4385)

crypto/asn1/a_bitstr.c

@@ -7,6 +7,7 @@
  * https://www.openssl.org/source/license.html
  */
 
+#include <limits.h>
 #include <stdio.h>
 #include "internal/cryptlib.h"
 #include <openssl/asn1.h>
@@ -88,6 +89,11 @@ ASN1_BIT_STRING *c2i_ASN1_BIT_STRING(ASN1_BIT_STRING **a,
         goto err;
     }
 
+    if (len > INT_MAX) {
+        i = ASN1_R_STRING_TOO_LONG;
+        goto err;
+    }
+
     if ((a == NULL) || ((*a) == NULL)) {
         if ((ret = ASN1_BIT_STRING_new()) == NULL)
             return (NULL);