Add conditional check for PDB location

File.OpenRead can throw if pdb.PdbLocation is a directory.

This can happen if (for some reason) the pdb file is not found.
In which case PdbLocation will return the working directory, causing
File.OpenRead to throw.

ReleaseHistory.md

@@ -16,6 +16,7 @@
 - NEW => new feature 
 
 ## UNRELEASED
+* BUG: Fix `TryGetPortablePdbMetadataReader` unexpectedly causes `UnauthorizedAccessException` error when the PDB file is missing. [1001](https://github.com/microsoft/binskim/pull/1001).
 
 ## **v4.3.0**
 * DEP: Update `msdia140.dll` from 14.36.32532.0 to 14.40.33810.0. This update fixes the `System.AccessViolationException: Attempted to read or write protected memory` exception that occurs when reading certain PDB files. [996](https://github.com/microsoft/binskim/pull/996)

src/BinaryParsers/PEBinary/PortableExecutable/PE.cs

@@ -868,7 +868,11 @@ private bool TryGetPortablePdbMetadataReader(Pdb pdb, out MetadataReader pdbMeta
                 out MetadataReaderProvider pdbProvider,
                 out _))
             {
-                pdbProvider = MetadataReaderProvider.FromPortablePdbStream(File.OpenRead(pdb.PdbLocation));
+                if (File.Exists(pdb.PdbLocation))
+                {
+                    pdbProvider = MetadataReaderProvider.FromPortablePdbStream(File.OpenRead(pdb.PdbLocation));
+                }
+
                 if (pdbProvider == null)
                 {
                     pdbMetadataReader = null;