time_interp_external2: Check field's existence before calling variable_att_exists in init_external_field

[J-Lentz]

variable_att_exists crashes when it is called on a nonexistent variable. Therefore, in init_external_field, the existence of the field should be checked before it is passed to variable_att_exists. This fix is required for the land model I/O API update.

Fixes #1192

How Has This Been Tested?
Compiles on the AMD box with Intel compiler.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included
• make distcheck passes