Add missing asserts to SSL_get_(min|max)_proto_version

It's a caller error to call those after the handshake config has been
shed. (We have an analogous pattern of asserts in other cases.)

Change-Id: Ifea0cb043a4f17e8e627d7fca81f90b8899c6587
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/78827
Auto-Submit: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/ssl_versions.cc b/ssl/ssl_versions.cc
index 4ef86e9..81b5fbf 100644
--- a/ssl/ssl_versions.cc
+++ b/ssl/ssl_versions.cc
@@ -386,6 +386,7 @@
 
 uint16_t SSL_get_min_proto_version(const SSL *ssl) {
   if (!ssl->config) {
+    assert(ssl->config);
     return 0;
   }
   return ssl->config->conf_min_version;
@@ -393,6 +394,7 @@
 
 uint16_t SSL_get_max_proto_version(const SSL *ssl) {
   if (!ssl->config) {
+    assert(ssl->config);
     return 0;
   }
   return ssl->config->conf_max_version;