[bug-libmatheval] Segmentation fault with 1.1.9

[Evan Andersen]

I can produce a segmentation fault in evaluator_evaluate() by entering the following input:

-5.45exp(-t/0.22)

However, adding a * between the constant and exp allows the code to run successfully. 

-5.45*exp(-t/0.22)

I check the output of evaluator_create(), and it does return a valid pointer in both instances. 

I have attached the code I was using to perform this test. (it's short)

--Evan

mathtest.c
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