Add assertions

Passing an unknown enum value to convert them to string would return
NULL without any error, possibly causing undefined behavior later.

Add assertions to catch such programming errors early.

app/src/server.c

@@ -147,7 +147,7 @@ log_level_to_server_string(enum sc_log_level level) {
             return "error";
         default:
             assert(!"unexpected log level");
-            return "(unknown)";
+            return NULL;
     }
 }
 
@@ -183,6 +183,7 @@ sc_server_get_codec_name(enum sc_codec codec) {
         case SC_CODEC_RAW:
             return "raw";
         default:
+            assert(!"unexpected codec");
             return NULL;
     }
 }
@@ -197,6 +198,7 @@ sc_server_get_camera_facing_name(enum sc_camera_facing camera_facing) {
         case SC_CAMERA_FACING_EXTERNAL:
             return "external";
         default:
+            assert(!"unexpected camera facing");
             return NULL;
     }
 }