display filter: the body of a range should only be
a string, a field name or another range - not an unparsed element

Bug: 10690
Change-Id: I126143636c940cc73ed6467660f0a573209e2ae9
Reviewed-on: https://code.wireshark.org/review/5243
Reviewed-by: Martin Kaiser <wireshark@kaiser.cx>
Tested-by: Martin Kaiser <wireshark@kaiser.cx>