git_libgit2_opts: validate key

src/settings.c

@@ -181,6 +181,9 @@ int git_libgit2_opts(int key, ...)
 		}
 
 		break;
+	default:
+		giterr_set(GITERR_INVALID, "invalid option key");
+		error = -1;
 	}
 
 	va_end(ap);

tests/core/opts.c

@@ -17,3 +17,9 @@ void test_core_opts__readwrite(void)
 
 	cl_assert(new_val == old_val);
 }
+
+void test_core_opts__invalid_option(void)
+{
+	cl_git_fail(git_libgit2_opts(-1, "foobar"));
+}
+