FindOpenSSL: Fix OpenSSL 3.0.0 version extraction

Fix the regex syntax added by commit 61d746e592 (FindOpenSSL: Detect
OpenSSL 3.0.0, 2020-05-27, v3.17.3~1^2).  Add missing escapes.
Test with `openssl-3.0.0-alpha3`.

While at it, also unset a temporary variable after use.

Modules/FindOpenSSL.cmake

@@ -419,11 +419,13 @@ if(OPENSSL_INCLUDE_DIR AND EXISTS "${OPENSSL_INCLUDE_DIR}/openssl/opensslv.h")
     # Since OpenSSL 3.0.0, the new version format is MAJOR.MINOR.PATCH and
     # a new OPENSSL_VERSION_STR macro contains exactly that
     file(STRINGS "${OPENSSL_INCLUDE_DIR}/openssl/opensslv.h" OPENSSL_VERSION_STR
-         REGEX "^#[\t ]*define[\t ]+OPENSSL_VERSION_STR[\t ]+\"([0-9])+\.([0-9])+\.([0-9])+\".*")
-    string(REGEX REPLACE "^.*OPENSSL_VERSION_STR[\t ]+\"([0-9]+\.[0-9]+\.[0-9]+)\".*$"
+         REGEX "^#[\t ]*define[\t ]+OPENSSL_VERSION_STR[\t ]+\"([0-9])+\\.([0-9])+\\.([0-9])+\".*")
+    string(REGEX REPLACE "^.*OPENSSL_VERSION_STR[\t ]+\"([0-9]+\\.[0-9]+\\.[0-9]+)\".*$"
            "\\1" OPENSSL_VERSION_STR "${OPENSSL_VERSION_STR}")
 
     set(OPENSSL_VERSION "${OPENSSL_VERSION_STR}")
+
+    unset(OPENSSL_VERSION_STR)
   endif ()
 endif ()