build fix for MONITOR_HEAP

Compile of alloc.c failed when MONITOR_HEAP is defined.

src/alloc.c

@@ -221,7 +221,7 @@ nhdupstr(const char *string, const char *file, int line)
     /* we've got some info about the caller, so use it instead of __func__ */
     unsigned len = FITSuint_(strlen(string), file, line);
 
-    if (FITSuint(len + 1, file, line) < len)
+    if (FITSuint_(len + 1, file, line) < len)
         panic("nhdupstr: string length overflow, line %d of %s",
               line, file);