Fix potential out-of-bounds read in json_tokener_error_desc

Found by Coverity. The number of elements of an array 'ar' is found by
sizeof(ar)/sizeof(ar[0]) and not sizeof(ar)

76const char *json_tokener_error_desc(enum json_tokener_error jerr)
 77{
 78        int jerr_int = (int)jerr;

1. Condition "jerr_int < 0", taking false branch

2. Condition "jerr_int > 112 /* (int)sizeof (gdal_json_tokener_errors) */", taking false branch
 79        if (jerr_int < 0 || jerr_int > (int)sizeof(json_tokener_errors))
 80                return "Unknown error, invalid json_tokener_error value passed to json_tokener_error_desc()";

CID 1076806 (#1 of 1): Out-of-bounds read (OVERRUN)3. overrun-local: Overrunning array "gdal_json_tokener_errors" of 14 8-byte elements at element index 112 (byte offset 896) using index "jerr" (which evaluates to 112).
 81        return json_tokener_errors[jerr];
 82}