muscles: fix handling of the "@'" escape

When we use `b4_` or `m4_` somewhere in the input, it is escaped as
`b4@'_`/`m4@'_` so that the warning about unexpanded b4_foo/m4_foo
macros does not fire.

But in the case of muscles, the `@'` escape was not recognized, and an
assertion was triggered.

Reported by Han Zheng.
<https://github.com/akimd/bison/issues/91>

* src/muscle-tab.c (COMMON_DECODE): Handle `@'`.
* tests/skeletons.at (Suspicious sequences): Check that case.
2 files changed