Add a helper function for resetting SSL_get_error state.

We repeat this in a bunch of places.

Change-Id: Iee2c95a13e1645453f101d8be4be9ac78d520387
Reviewed-on: https://boringssl-review.googlesource.com/13051
Reviewed-by: David Benjamin <davidben@google.com>

ssl/internal.h

diff --git a/ssl/internal.h b/ssl/internal.h
index 97ae6a0..e26fa13 100644
--- a/ssl/internal.h
+++ b/ssl/internal.h
@@ -1946,6 +1946,9 @@
 
 void ssl_get_current_time(const SSL *ssl, struct timeval *out_clock);
 
+/* ssl_reset_error_state resets state for |SSL_get_error|. */
+void ssl_reset_error_state(SSL *ssl);
+
 
 #if defined(__cplusplus)
 } /* extern C */