gvariant-parser: Fix error handling when type coalescing fails

When parsing GVariant text format strings, we do a limited form of type inference. The algorithm for type inference for nested array child types is not complete, however (and making it complete, at least with a naive implementation, would make it O(N^2), which is not worth it) and so some text format arrays were triggering an assertion failure in the error handling code.

Fix that by making the error handling code a little more relaxed, in the knowledge that our type inference algorithm is not complete. See the comment added to the code.

This includes a test case, provided by oss-fuzz.

oss-fuzz#11578

Signed-off-by: Philip Withnall withnall@endlessm.com