Fix SSL_CTX_set1_sigalgs_list fuzzer.

SSL_CTX_set1_sigalgs_list wants a NUL-terminated string, so we need to use
GetString to give it one.

Bug: oss-fuzz:9808
Change-Id: Id7f676aa514c36de9dea900763db3cbbf5c79a4c
Reviewed-on: https://boringssl-review.googlesource.com/30804
Reviewed-by: Adam Langley <agl@google.com>

fuzz/ssl_ctx_api.cc

diff --git a/fuzz/ssl_ctx_api.cc b/fuzz/ssl_ctx_api.cc
index c050770..4b47734 100644
--- a/fuzz/ssl_ctx_api.cc
+++ b/fuzz/ssl_ctx_api.cc
@@ -479,7 +479,11 @@
         SSL_CTX_set1_sigalgs(ctx, (const int *)CBS_data(cbs), CBS_len(cbs) / 2);
       },
       [](SSL_CTX *ctx, CBS *cbs) {
-        SSL_CTX_set1_sigalgs_list(ctx, (const char *) CBS_data(cbs));
+        std::string sigalgs;
+        if (!GetString(&sigalgs, cbs)) {
+          return;
+        }
+        SSL_CTX_set1_sigalgs_list(ctx, sigalgs.c_str());
       },
   };