qemu_conf: Don't discard strdup OOM error

After 78d7c3c5 we are strdup()-ing path to qemu-bridge-helper.
However, the check for its return value is missing. So it is
possible we've ignored the OOM error silently.

src/qemu/qemu_conf.c

@@ -241,7 +241,8 @@ virQEMUDriverConfigPtr virQEMUDriverConfigNew(bool privileged)
         }
     }
 #endif
-    cfg->bridgeHelperName = strdup("/usr/libexec/qemu-bridge-helper");
+    if (!(cfg->bridgeHelperName = strdup("/usr/libexec/qemu-bridge-helper")))
+        goto no_memory;
 
     cfg->clearEmulatorCapabilities = true;