Cygwin: cygheap: Fix the issue of cygwin1.dll in the root directory.

- After the commit 6d898f43, cygwin fails to start if cygwin1.dll
  is placed in the root directory. This patch fixes the issue.
Addresses: https://cygwin.com/pipermail/cygwin/2022-May/251548.html

winsup/cygwin/cygheap.cc

@@ -183,6 +183,11 @@ init_cygheap::init_installation_root ()
 	  if (p)
 	    p = wcschr (p + 1, L'\\');  /* Skip share name */
 	}
+      else /* Long path prefix followed by drive letter path */
+	{
+	  len = 4;
+	  p += 4;
+	}
     }
   installation_root_buf[1] = L'?';
   RtlInitEmptyUnicodeString (&installation_key, installation_key_buf,