Fix incorrect upper bound of RTP payload validity check (#946)

[hogliux]

This PR fixes #946 by fixing the upper limits of the conditional in lib/src/mt_util.h:92 and app/src/parse_json.c:467.

[Sakoram]

Due to temporary infrastructure problems, I ran the unit tests locally. They passed successfully:

Running test: ===========================================
./build/tests/KahawaiUfdTest --p_port 0000:4b:01.1 --r_port 0000:4b:01.2

Test passed in 17 seconds.

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Running test: ===========================================
./build/tests/KahawaiUfdTest --p_port 0000:4b:01.1 --r_port 0000:4b:01.2 --queue_mode shared

Test passed in 18 seconds.

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Running test: ===========================================
./build/tests/KahawaiUfdTest --p_port 0000:4b:01.1 --r_port 0000:4b:01.2 --queue_mode shared --udp_lcore

Test passed in 19 seconds.

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Running test: ===========================================
./build/tests/KahawaiUfdTest --p_port 0000:4b:01.1 --r_port 0000:4b:01.2 --rss_mode l3_l4

Test passed in 18 seconds.

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Running test: ===========================================
./build/tests/KahawaiTest --auto_start_stop --p_port 0000:4b:01.1 --r_port 0000:4b:01.2 --dma_dev 0000:00:01.0,0000:00:01.1 --gtest_filter=-St22_?x.*

Test passed in 786 seconds.

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Running test: ===========================================
./build/tests/KahawaiTest --auto_start_stop --p_port 0000:4b:01.1 --r_port 0000:4b:01.2 --rss_mode l3_l4 --pacing_way tsc --iova_mode pa --multi_src_port --gtest_filter=Main.:St20p:-ext

Test passed in 80 seconds.

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Running test: ===========================================
./build/tests/KahawaiTest --p_port kernel:lo --r_port kernel:lo --auto_start_stop --gtest_filter=St20p*

Test passed in 138 seconds.

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