virTestSetEnvPath: Avoid clearing out PATH

If the abs_builddir path already is in PATH and it's in the first
position, due to a bug in our code PATH would be cleared out.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tests/testutils.c

@@ -822,7 +822,8 @@ virTestSetEnvPath(void)
             goto cleanup;
     }
 
-    if (setenv("PATH", new_path, 1) < 0)
+    if (new_path &&
+        setenv("PATH", new_path, 1) < 0)
         goto cleanup;
 
     ret = 0;