Fix data race in asn1_str2tag() on tntmp which was accidentally made static

Fix data race in asn1_str2tag() on tntmp which was accidentally made static

Variables tntmp and tnst are declared in the same declaration and thus
share storage class specifiers (static). This is unfortunate as tntmp is
used during iteration through tnst array and shouldn't be static.
In particular this leads to two problems that may arise when multiple
threads are executing asn1_str2tag() concurrently:
1. asn1_str2tag() might return value that doesn't correspond to tagstr
   parameter. This can happen if other thread modifies tntmp to point to
   a different tnst element right after a successful name check in the
   if statement.
2. asn1_str2tag() might perform an out-of-bounds read of tnst array.
   This can happen when multiple threads all first execute tntmp = tnst;
   line and then start executing the loop. If that case those threads
   can end up incrementing tntmp past the end of tnst array.

CLA: trivial

Reviewed-by: Paul Dale <ppzgs1@gmail.com>
Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/26504)