ostree: check g_setenv return value

This adds proper return-value checks on g_setenv calls.
It fixes a static analysis warning highlighted by Coverity.

tests/libostreetest.c

@@ -155,7 +155,8 @@ ot_test_setup_sysroot (GCancellable *cancellable,
     }
 
   /* Make sure deployments are mutable */
-  g_setenv ("OSTREE_SYSROOT_DEBUG", buf->str, TRUE);
+  if (!g_setenv ("OSTREE_SYSROOT_DEBUG", buf->str, TRUE))
+    return glnx_null_throw (error, "Failed to set environment variable OSTREE_SYSROOT_DEBUG");
 
   g_autoptr(GFile) sysroot_path = g_file_new_for_path ("sysroot");
   return ostree_sysroot_new (sysroot_path);

tests/test-gpg-verify-result.c

@@ -78,7 +78,8 @@ test_fixture_setup (TestFixture *fixture,
    * certificates for certain test cases. */
 
   homedir = g_test_build_filename (G_TEST_DIST, "tests/gpg-verify-data", NULL);
-  g_setenv ("GNUPGHOME", homedir, TRUE);
+  gboolean is_ok = g_setenv ("GNUPGHOME", homedir, TRUE);
+  g_assert (is_ok == TRUE);
 
   result = g_initable_new (OSTREE_TYPE_GPG_VERIFY_RESULT,
                            NULL, &local_error, NULL);

tests/test-rollsum-cli.c

@@ -35,7 +35,8 @@ main (int argc, char **argv)
   OstreeRollsumMatches *matches;
   GMappedFile *mfile;
 
-  g_setenv ("GIO_USE_VFS", "local", TRUE);
+  gboolean is_ok = g_setenv ("GIO_USE_VFS", "local", TRUE);
+  g_assert (is_ok == TRUE);
 
   if (argc < 3)
     return 1;

tests/test-varint.c

@@ -58,7 +58,8 @@ int
 main (int argc, char **argv)
 {
 
-  g_setenv ("GIO_USE_VFS", "local", TRUE);
+  gboolean is_ok = g_setenv ("GIO_USE_VFS", "local", TRUE);
+  g_assert (is_ok == TRUE);
 
   g_test_init (&argc, &argv, NULL);