Fix uninitialised return value.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>

programs/fuzz/common.c

@@ -64,6 +64,9 @@ int dummy_random( void *p_rng, unsigned char *output, size_t output_len )
     if( p_rng != NULL ) {
         //use mbedtls_ctr_drbg_random to find bugs in it
         ret = mbedtls_ctr_drbg_random(p_rng, output, output_len);
+    } else {
+        //fall through to pseudo-random
+        ret = 0;
     }
 #else
     (void) p_rng;