[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

Author: richlowe

compiler-rt/lib/sanitizer_common/sanitizer_procmaps_solaris.cpp

@@ -55,7 +55,15 @@ bool MemoryMappingLayout::Next(MemoryMappedSegment *segment) {
 
     internal_snprintf(proc_path, sizeof(proc_path), "/proc/self/path/%s",
                       xmapentry->pr_mapname);
-    internal_readlink(proc_path, segment->filename, segment->filename_size);
+    ssize_t sz = internal_readlink(proc_path, segment->filename,
+                                   segment->filename_size - 1);
+
+    // If readlink failed, the map is anonymous.
+    if (sz == -1) {
+      segment->filename[0] = '\0';
+    } else if ((size_t)sz < segment->filename_size)
+      // readlink doesn't NUL-terminate.
+      segment->filename[sz] = '\0';
   }
 
   data_.current += sizeof(prxmap_t);