expressions: Parse multiple sets of parentheses for grouping together.

Fuzzers are fond of driving expression parsers to failure by exhausting
the stack in trivial ways.  This defeats the simplest attempts by
lining up thousands of left parentheses in a row.

I am a bit curious whether the fuzzer will now invent something more
sophisticated, such as nested function calls or non-empty expressions like
1+(1+(1+(1+(1+....

This fixes bug #61286.
Thanks to Irfan Ariq for reporting the bug.