[SV 65172] Avoid buffer overruns when expanding for $(shell ...)

Reported-by: MIAOW Miao <guoyr_2013@hotmail.com>
Patch from: Henrik Carlqvist <hc981@poolhem.se>
Test from: Dmitry Goncharov <dgoncharov@users.sf.net>

* src/expand.c (recursively_expand_for_file): Check the variable name
before checking for equality so we don't overrun the buffer.
* tests/scripts/functions/shell: Add a test with a very long variable.