Fix uninitialized variable in AVR test runner

Caused crash on newer simavr versions.
diff --git a/tests/site_scons/platforms/avr/run_test.c b/tests/site_scons/platforms/avr/run_test.c
index 8e4679e..eca0d32 100644
--- a/tests/site_scons/platforms/avr/run_test.c
+++ b/tests/site_scons/platforms/avr/run_test.c
@@ -146,7 +146,7 @@
         filename = argv[2];
     }
     
-    elf_firmware_t firmware;
+    elf_firmware_t firmware = {};
     elf_read_firmware(filename, &firmware);
     avr_init(g_avr);
 	avr_load_firmware(g_avr, &firmware);