Fix buggy gen_simple_symbol.

gen_symbol must guarantee that the first symbol character is not a minus
or plus if the next character COULD be a digit.  In the first list of
the oneof clause, gen_simple_symbol explicitly allows minus and plus as
the first character and follows that by a non-numeric list, which is ok,
but in the second list of the oneof clause, it specifies a list of
gen_non_numeric - which COULD be a minus or plus. Following the oneof
clause, the gen_symbol_char COULD be a digit, which produces an invalid
symbol.

Adding one character from a generator that explicitly excludes minus and
plus seems to fix the issue, at least when I ran 10000 test iterations.
I was able to get the commit prior to this one to fail within 10000
tests.  However, I am not sure how the change affects the overall
testing, maybe by excluding some valid test cases. Also, I don't have a
QC license, so I used QC Mini, which may or may not be a fair
substitution.