alloc/test: Fix trx_no assertion

Currently the value of trx_no2 is used in the assertion, even if the
call to tfi_find_free has failed.

This commit fixes the asserted expression to only compare the trx_no
values if the function call has succeeded.

Sponsored-by: On-Waves ehf
diff --git a/tests/alloc/AllocTest.cpp b/tests/alloc/AllocTest.cpp
index ac67ce9..429f588 100644
--- a/tests/alloc/AllocTest.cpp
+++ b/tests/alloc/AllocTest.cpp
@@ -633,7 +633,8 @@
 			tfi2 = the_bts->tfi_find_free(dir, &trx_no2,
 				ms->current_trx()->trx_no);
 			OSMO_ASSERT(tfi != tfi2);
-			OSMO_ASSERT(trx_no2 == ms->current_trx()->trx_no);
+			OSMO_ASSERT(tfi2 < 0 ||
+				trx_no2 == ms->current_trx()->trx_no);
 		}
 
 		ms_class += 1;