tests/fuzz: guard SCCalloc result in fuzz_decodebase64

If SCCalloc fails, decoded is NULL and the subsequent SCBase64Decode
call would dereference it. Return early on allocation failure.

src/tests/fuzz/fuzz_decodebase64.c

@@ -19,6 +19,8 @@ static void Base64FuzzTest(const uint8_t *src, size_t len)
 {
     uint32_t decoded_len = SCBase64DecodeBufferSize((uint32_t)len);
     uint8_t *decoded = SCCalloc(decoded_len, sizeof(uint8_t));
+    if (decoded == NULL)
+        return;
 
     for (uint8_t mode = SCBase64ModeRFC2045; mode <= SCBase64ModeStrict; mode++) {
         (void)SCBase64Decode(src, len, mode, decoded);