Use '== -1' to check for errors from functions.

Use '== -1' to check for errors from functions.

As made obvious by commit 123f53f, the type of ssize_t cannot be trusted
on Windows to actually be signed, so don't use '< 0', as the compiler
will optimize the test away. POSIX also specifies only that -1 signals
an error condition, not any other negative value.

Luckily, comparing an unsigned int to -1 still works as intended.