FindOpenSSL: Do not assume that the version regex finds something

BoringSSL's openslv.h does not have the version information.

Modules/FindOpenSSL.cmake

@@ -328,11 +328,11 @@ function(from_hex HEX DEC)
   set(${DEC} ${_res} PARENT_SCOPE)
 endfunction()
 
-if (OPENSSL_INCLUDE_DIR)
-  if(OPENSSL_INCLUDE_DIR AND EXISTS "${OPENSSL_INCLUDE_DIR}/openssl/opensslv.h")
-    file(STRINGS "${OPENSSL_INCLUDE_DIR}/openssl/opensslv.h" openssl_version_str
-         REGEX "^#[\t ]*define[\t ]+OPENSSL_VERSION_NUMBER[\t ]+0x([0-9a-fA-F])+.*")
+if(OPENSSL_INCLUDE_DIR AND EXISTS "${OPENSSL_INCLUDE_DIR}/openssl/opensslv.h")
+  file(STRINGS "${OPENSSL_INCLUDE_DIR}/openssl/opensslv.h" openssl_version_str
+       REGEX "^#[\t ]*define[\t ]+OPENSSL_VERSION_NUMBER[\t ]+0x([0-9a-fA-F])+.*")
 
+  if(openssl_version_str)
     # The version number is encoded as 0xMNNFFPPS: major minor fix patch status
     # The status gives if this is a developer or prerelease and is ignored here.
     # Major, minor, and fix directly translate into the version numbers shown in