detect/analyzer: address format truncation warning

detect-engine-analyzer.c: In function ‘SetupEngineAnalysis’:
detect-engine-analyzer.c:493:60: error: ‘%s’ directive output may be truncated writing up to 63 bytes into a region of size 3 [-Werror=format-truncation=]
  493 |         snprintf(ea->file_prefix, cfg_prefix_len + 1 + 1, "%s.", de_ctx->config_prefix);
      |                                                            ^~
detect-engine-analyzer.c:493:9: note: ‘snprintf’ output between 2 and 65 bytes into a destination of size 3
  493 |         snprintf(ea->file_prefix, cfg_prefix_len + 1 + 1, "%s.", de_ctx->config_prefix);
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Ticket: #7905.

src/detect-engine-analyzer.c

@@ -484,13 +484,12 @@ void SetupEngineAnalysis(DetectEngineCtx *de_ctx, bool *fp_analysis, bool *rule_
     ea->file_prefix = NULL;
     size_t cfg_prefix_len = strlen(de_ctx->config_prefix);
     if (cfg_prefix_len > 0) {
-        /* length of prefix + NULL + "." */
-        ea->file_prefix = SCCalloc(1, cfg_prefix_len + 1 + 1);
+        char prefix[sizeof(de_ctx->config_prefix) + 1];
+        snprintf(prefix, sizeof(prefix), "%s.", de_ctx->config_prefix);
+        ea->file_prefix = SCStrdup(prefix);
         if (ea->file_prefix == NULL) {
             FatalError("Unable to allocate per-engine analysis context name buffer");
         }
-
-        snprintf(ea->file_prefix, cfg_prefix_len + 1 + 1, "%s.", de_ctx->config_prefix);
     }
 
     de_ctx->ea = ea;