fix issues found by Coverity Scan in tscSQLParser.c (branch: 2.6)

[xiaopingcs]

Bug Description
fix issues found by Coverity Scan in TDengine/src/client/src/tscSQLParser.c

To Reproduce
N/A

Expected Behavior
fix following issues:

Type	Category	Function	Line Number
Out-of-bounds access	Memory - corruptions	doValidateSubquery	Various
Logically dead code	Control flow issues	setKeepOption	Various
Time of check time of use	Security best practices violations	readFromFile	433
Logically dead code	Control flow issues	getTimeRange	6759
Resource leak	Resource leaks	handleUserDefinedFunc	523
Unsigned compared against 0	Control flow issues	validateSQLExprItem	5309
Improper use of negative value	Integer handling issues	addExprAndResultField	2950
Copy-paste error	Incorrect expression	checkAndSetJoinCondInfo	4992
Unchecked return value	Error handling issues	parseSlidingClause	1490
Explicit null dereferenced	Null pointer dereferences	exprTreeFromSqlExpr	10972
Logically dead code	Control flow issues	validateSQLExprItemSQLFunc	5193
Use after free	Memory - illegal accesses	validateWhereNode	6624
Improper use of negative value	Integer handling issues	handleExprInQueryCond	Various

Screenshots
N/A

Environment (please complete the following information):
N/A

Additional Context
above issues found by https://scan.coverity.com/projects/tdengine?tab=overview