detect/content-inspection: assert smd is set

DetectEngineContentInspectionInternal checked smd for NULL only after
the recursion-limit return had read smd->type, and its NULL branch read
smd->type too. No caller passes NULL: engines without a keyword list
have no callback, empty sticky buffers fail to load, payload engines
exist only with payload keywords, and the recursion stops at the last
keyword.

Drop smd from the NULL check and assert it with DEBUG_VALIDATE_BUG_ON.

src/detect-engine-content-inspection.c

@@ -110,6 +110,9 @@ static int DetectEngineContentInspectionInternal(DetectEngineThreadCtx *det_ctx,
         const enum DetectContentInspectionType inspection_mode)
 {
     SCEnter();
+    // every caller passes a keyword list, and the recursion below stops at
+    // its last entry; the profiling calls rely on that to read smd->type
+    DEBUG_VALIDATE_BUG_ON(smd == NULL);
     KEYWORD_PROFILING_START;
 
     ctx->recursion.count++;
@@ -119,7 +122,7 @@ static int DetectEngineContentInspectionInternal(DetectEngineThreadCtx *det_ctx,
     }
 
     // we want the ability to match on bsize: 0
-    if (smd == NULL || buffer == NULL) {
+    if (buffer == NULL) {
         KEYWORD_PROFILING_END(det_ctx, smd->type, 0);
         SCReturnInt(0);
     }