fix: test-ranap.c: missing NULL init

The decoding target pointer has to be NULL so that a new structure is allocated
by aper_decode(). Fixes a probable segmentation fault upon decoding at
test-ranap.c:77 or free of unallocated memory in test-ranap.c:81.
diff --git a/src/tests/test-ranap.c b/src/tests/test-ranap.c
index d00f583..7b1f3bf 100644
--- a/src/tests/test-ranap.c
+++ b/src/tests/test-ranap.c
@@ -57,7 +57,7 @@
 static void test_aper_causemisc(uint32_t inp, uint8_t exp_enc)
 {
 	RANAP_Cause_t c = { .present = RANAP_Cause_PR_misc, .choice.misc = inp };
-	RANAP_Cause_t *c_dec;
+	RANAP_Cause_t *c_dec = NULL;
 	asn_enc_rval_t rv;
 	uint8_t buf[32];