detect: fix memory leak in error handling

Pointed out by cppcheck:

src/detect-parse.c:1544:9: error: Memory leak: sig.init_data [memleak]
        SCFree(sig);
        ^

Bug: #6527.

src/detect-parse.c

@@ -1541,6 +1541,7 @@ Signature *SigAlloc (void)
 
     sig->init_data->buffers = SCCalloc(8, sizeof(SignatureInitDataBuffer));
     if (sig->init_data->buffers == NULL) {
+        SCFree(sig->init_data);
         SCFree(sig);
         return NULL;
     }