replace strstr() with strchr() for single characters

replace strstr() with strchr() for single characters

strstr() is used to match multiple characters in the haystack,
whereas strchr() is used to matched only single character.

CLA: trivial

Reviewed-by: Tom Cosgrove <tom.cosgrove@arm.com>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23347)