[fix](jni-connector) avoid core dump if init connector failed

[morningman]

Proposed changes

_jni_scanner_cls may be null if connector init failed.
So need to check it before delete it.

Further comments

If this is a relatively large or complex change, kick off the discussion at dev@doris.apache.org by explaining why you chose the solution you did and what alternatives you considered, etc...

[doris-robot]

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[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[dataroaring]

LGTM

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"

[yiguolei]

LGTM

[yiguolei]

run buildall

[doris-robot]

TeamCity be ut coverage result:
Function Coverage: 35.24% (8916/25304)
Line Coverage: 27.00% (73331/271645)
Region Coverage: 26.17% (37882/144747)
Branch Coverage: 22.98% (19291/83932)
Coverage Report: http://coverage.selectdb-in.cc/coverage/4046ab2ba7a3f392dab5673649d534e3ab8d5e9f_4046ab2ba7a3f392dab5673649d534e3ab8d5e9f/report/index.html

[doris-robot]

ClickBench: Total hot run time: 31.58 s

machine: 'aliyun_ecs.c7a.8xlarge_32C64G'
scripts: https://github.com/apache/doris/tree/master/tools/clickbench-tools
ClickBench test result on commit 4046ab2ba7a3f392dab5673649d534e3ab8d5e9f, data reload: false

query1	0.04	0.03	0.03
query2	0.08	0.04	0.04
query3	0.23	0.05	0.05
query4	1.67	0.08	0.09
query5	0.50	0.51	0.50
query6	1.25	0.87	0.81
query7	0.02	0.02	0.01
query8	0.06	0.04	0.04
query9	0.52	0.46	0.45
query10	0.52	0.51	0.50
query11	0.15	0.10	0.11
query12	0.14	0.11	0.11
query13	0.63	0.63	0.63
query14	0.92	1.01	1.07
query15	0.85	0.85	0.85
query16	0.37	0.37	0.38
query17	1.04	1.03	1.04
query18	0.21	0.22	0.27
query19	1.87	1.81	1.81
query20	0.02	0.01	0.02
query21	15.40	0.65	0.65
query22	3.87	8.13	1.93
query23	18.31	1.33	1.35
query24	1.88	0.29	0.25
query25	0.16	0.10	0.10
query26	0.27	0.18	0.18
query27	0.09	0.08	0.09
query28	13.31	1.04	1.01
query29	12.65	3.42	3.36
query30	0.25	0.06	0.06
query31	2.85	0.41	0.41
query32	3.22	0.48	0.48
query33	2.73	2.77	2.78
query34	17.22	4.56	4.55
query35	4.54	4.61	4.57
query36	0.66	0.46	0.46
query37	0.21	0.19	0.19
query38	0.21	0.20	0.19
query39	0.06	0.04	0.05
query40	0.18	0.15	0.15
query41	0.11	0.06	0.06
query42	0.06	0.06	0.07
query43	0.05	0.05	0.05
Total cold run time: 109.38 s
Total hot run time: 31.58 s