In `find_common_machine_version`, the code previously assumed that
`getenv(var1)` and `getenv(var2)` would always return non-NULL values.
However, if either environment variable is not set, `getenv` returns
NULL, which could lead to a null pointer dereference.

Tracing upstream usage: `find_common_machine_version` is called by
`resolve_machine_version` with `QEMU_ENV_SRC` and `QEMU_ENV_DST`.
`resolve_machine_version` is used by `migrate_start`, which is called
by `migrate_postcopy_prepare`, and ultimately by `test_postcopy_common`.

In `test_postcopy_common`, after `migrate_postcopy_prepare`, the
function `migrate_postcopy_complete` is called. Inside, 
`migration_get_env` checks if `QEMU_ENV_SRC` and `QEMU_ENV_DST` are
set before use. Thus, these variables can be NULL, leading to a
potential null pointer dereference in `find_common_machine_version`.

Signed-off-by: xjdeng <micro6947@gmail.com>
---
 tests/qtest/migration/migration-util.c | 21 +++++++++++++++++++--
 1 file changed, 19 insertions(+), 2 deletions(-)

diff --git a/tests/qtest/migration/migration-util.c b/tests/qtest/migration/migration-util.c
index 642cf50c8d..45c9e164e2 100644
--- a/tests/qtest/migration/migration-util.c
+++ b/tests/qtest/migration/migration-util.c
@@ -203,8 +203,25 @@ char *find_common_machine_version(const char *mtype, const char *var1,
         return g_strdup(type2);
     }
 
-    g_test_message("No common machine version for machine type '%s' between "
-                   "binaries %s and %s", mtype, getenv(var1), getenv(var2));
+    char *varstring1 = getenv(var1);
+    char *varstring2 = getenv(var2);
+    if (varstring1 && varstring2) {
+        g_test_message("No common machine version for machine type '%s' "
+                       "between binaries %s and %s",
+                       mtype, varstring1, varstring2);
+    } else if (varstring1) {
+        g_test_message("No common machine version for machine type '%s' "
+                       "between binary %s and environment variable %s",
+                       mtype, varstring1, var2);
+    } else if (varstring2) {
+        g_test_message("No common machine version for machine type '%s' "
+                       "between binary %s and environment variable %s",
+                       mtype, varstring2, var1);
+    } else {
+        g_test_message("No common machine version for machine type '%s' "
+                       "between environment variables %s and %s",
+                       mtype, var1, var2);
+    }
     g_assert_not_reached();
 }
 
-- 
2.27.0.windows.1