[sanitizer_common] Fix readlink error handling in sanitizer_procmaps_solaris.cpp

As pointed out in Bug 52371, the Solaris version of
`MemoryMappingLayout::Next` completely failed to handle `readlink` errors
or properly NUL-terminate the result.

This patch fixes this.  Originally provided in the PR with slight
formatting changes.

Tested on `amd64-pc-solaris2.11`.

Differential Revision: https://reviews.llvm.org/D112998