Signed down cast negates usefulness of preceding bounds check

[pgoodman]

Describe the bug
The following code checks that strtol doesn't read a negative number from the environment variable.

What the code should really do is check that ((int16_t) nthreads) > 0. That is, strtol could read in 65535, which is a non-negative long number. Then, the (highlighted) cast to an int16_t would interpret this as -1.

To Reproduce
Set BLOSC_NTHREADS environment variable to 65535.

System information:

• OS: macOS
• Compiler: Clang
• Version: 16