[efi] Avoid modifying PE/COFF debug filename

The function efi_pecoff_debug_name() (called by efi_handle_name()) is
used to extract a filename from the debug data directory entry located
within a PE/COFF image.  The name is copied into a temporary static
buffer to allow for modifications, but the code currently erroneously
modifies the original name within the loaded PE/COFF image.

Fix by performing the modification on the copy in the temporary
buffer, as originally intended.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/interface/efi/efi_debug.c

@@ -665,10 +665,10 @@ efi_pecoff_debug_name ( EFI_LOADED_IMAGE_PROTOCOL *loaded ) {
 	snprintf ( buf, sizeof ( buf ), "%s", name );
 
 	/* Strip file suffix, if present */
-	if ( ( tmp = strrchr ( name, '.' ) ) != NULL )
+	if ( ( tmp = strrchr ( buf, '.' ) ) != NULL )
 		*tmp = '\0';
 
-	return name;
+	return buf;
 }
 
 /**