series not working with symbolic functions

[jmsureda]

series does not seem to work with symbolic functions. As shown in the code below, when including a symbolic function in the expression given to series it kills the kernel (in Jupiter notebook) and produces a segmentation fault. This error appears on multiple expressions and I've identified the issue to be produced by this kind of operations.

`import symengine as se
t, x = se.symbols("t x")
f = se.Function("f")(x)

print(se.series(se.cos(t) ,t, 0, 3))

print(se.series(se.cos(t) * f, t, 0, 3))`