app: fix Windows app-path resolution

Fix another bug in how we compute the Application Executable path on
Windows. It some cases we can still get a relative/non-absolute path to
the entry executable. If we get a filename/non-path from the native API
calls then return null and fallback to using process/module image
resolution instead. For .NET Framework on Windows, this is OK.
We will want to revisit this on .NET (Core) on Windows, if and when we
get around to that...

Author: mjcheetham

src/shared/Core/PlatformUtils.cs

@@ -289,7 +289,11 @@ private static string GetWindowsEntryPath()
             IntPtr argv0Ptr = Marshal.ReadIntPtr(argvPtr);
             string argv0 = Marshal.PtrToStringAuto(argv0Ptr);
             Interop.Windows.Native.Kernel32.LocalFree(argvPtr);
-            return argv0;
+
+            // If this isn't absolute then we should return null to prevent any
+            // caller that expect only an absolute path from mis-using this result.
+            // They will have to fall-back to other mechanisms for getting the entry path.
+            return Path.IsPathRooted(argv0) ? argv0 : null;
         }
 
         #endregion