Fix #687, validate table handle prior to use

[CDKnightNASA]

Describe the contribution
fixes a bug where a table handle is used prior to validation (used in the variable initialization at the top of CFE_TBL_Load() .)

Fix #687

Contributor Info - All information REQUIRED for consideration of pull request
[email protected]

[skliper]

Squash these (just don't assign NULL):

[fsw/cfe-core/src/tbl/cfe_tbl_api.c:681] -> [fsw/cfe-core/src/tbl/cfe_tbl_api.c:689]: (style) Variable 'AccessDescPtr' is reassigned a value before the old one has been used.
[fsw/cfe-core/src/tbl/cfe_tbl_api.c:682] -> [fsw/cfe-core/src/tbl/cfe_tbl_api.c:690]: (style) Variable 'RegRecPtr' is reassigned a value before the old one has been used.
[fsw/cfe-core/src/tbl/cfe_tbl_api.c:680] -> [fsw/cfe-core/src/tbl/cfe_tbl_api.c:696]: (style) Variable 'WorkingBufferPtr' is reassigned a value before the old one has been used.

[CDKnightNASA]

updated, squashed, pushed, better? :)

[skliper]

CCB 20200513 - Approved

Changes applied