Harden the lower level parts of crypto/asn1 against overflows.

The legacy ASN.1 stack contains an unsalvageable mix of integer types.
82dfea8d9e bounded all inputs to the template
machinery, but sometimes code will call ASN1_get_object directly, such as the
just deleted d2i_ASN1_UINTEGER.

Thanks to mlbrown for reporting the d2i_ASN1_UINTEGER overflow.

Bug: chromium:942269
Change-Id: I2d4c8b7faf5dadd1b68dbdb51a5feae071ea2cb6
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/35325
Reviewed-by: Adam Langley <agl@google.com>

crypto/asn1/asn1_lib.c

@@ -205,7 +205,11 @@ static int asn1_get_length(const unsigned char **pp, int *inf, long *rl,
         } else
             ret = i;
     }
-    if (ret > LONG_MAX)
+    /*
+     * Bound the length to comfortably fit in an int. Lengths in this module
+     * often switch between int and long without overflow checks.
+     */
+    if (ret > INT_MAX / 2)
         return 0;
     *pp = p;
     *rl = (long)ret;