fix for last commit

Author: jacobwilliams

src/function_parser.f90

@@ -1804,6 +1804,7 @@ function completely_enclosed (f, b, e) result (res)
     integer :: j,k
 
     res=.false.
+    if (b==0 .or. e==0) return
     if (f(b:b) == '(' .and. f(e:e) == ')') then
         k = 0
         do j=b+1,e-1