detect/mpm: fix fast_pattern handling of len >255

The fast pattern selection logic would truncate a patterns len to
255 leading to assigning the same pid to different patterns.

This in turn would be caught by the hyperscan setup code which would
abort.

Bug #2714.

src/detect-engine-mpm.c

@@ -1505,7 +1505,7 @@ int DetectSetFastPatternAndItsId(DetectEngineCtx *de_ctx)
         return -1;
 
     uint8_t *content = NULL;
-    uint8_t content_len = 0;
+    uint16_t content_len = 0;
     PatIntId max_id = 0;
     DetectFPAndItsId *struct_offset = (DetectFPAndItsId *)ahb;
     uint8_t *content_offset = ahb + struct_total_size;