Ensure the buffer provided to MPAS_io_get_var_generic is large enough.

[jim-p-w]

This PR fixes a potential buffer overrun when reading string variables from a netcdf file.

A fixed size array is provided as an output buffer when reading a 0d-char character variable.
Call MPAS_io_inq_var prior to the read to get the size of the variable, and only proceed with the read if the size of the variable will fit in the provided array.
Return an error code if the variable value is larger than the provided output buffer.

A unit test is included to verify:

1. An attempt to read a string variable into a buffer which is too small to hold the string value is detected
2. When a buffer which is too small is detected, the read won't occur and an error code is returned
3. When the test is run via valgrind, valgrind detects no memory errors or corruption
4. If the code which detects the too small buffer is commented out the test crashes and valgrind reports memory corruption

Fixes issue #1350

Note

When building with PIO, if the charArray (or the charArray1d) value exceeds the size of the tempchar buffer provided to the call to PIO_get_var, the value will be truncated to the size of the provided tempchar buffer (lines 2018, 2023, and 2042 in src/framework/mpas_io.F).

[mgduda]

@jim-p-w I've verified that the changes to mpas_io.F catch cases where we would otherwise have tried to read a string into a buffer that was too small. If you could rework the commit history so that it contains just a couple of commits -- perhaps one to modify the framework mpas_io code, and another to add unit tests -- I think we'll be ready to merge this.

[jim-p-w]

@mgduda Is the initial commit message in the PR suitable for the framework commit? Or do you have suggestions/edits you would prefer to see?