sancov.py: [Py3] Use bytes.decode() explicitly.

Or bogus filename like "b'foo'" would be generated.

This is part of https://reviews.llvm.org/D27405

llvm-svn: 294307

compiler-rt/lib/sanitizer_common/scripts/sancov.py

@@ -113,7 +113,7 @@ def UnpackOneFile(path):
       if len(header) < 12:
         break
       pid, module_length, blob_size = struct.unpack('iII', header)
-      module = f.read(module_length)
+      module = f.read(module_length).decode('utf-8')
       blob = f.read(blob_size)
       assert(len(module) == module_length)
       assert(len(blob) == blob_size)