fix miscellaneous compiler warnings
The attached patch tweaks things to avoid miscellaneous compiler warnings, from GCC and from the Clang static analyzer:
- Avoid signed/unsigned comparison with judicious casts, justified by asserts.
- Use the same type for indices into an array as are used for the size of the array.
- Make sure variables are initialized even for error branches.
- Avoid possible null pointer dereference in case of test failure.
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