Fix compilation error of test w/ GCC-9

 - GFortran 9.1 complains about initializing a precedure pointer to an internal procedure
 - ¯\_(ツ)_/¯

Author: zbeekman

src/tests/integration/pde_solvers/coarrayBurgers/main.F90

@@ -8,7 +8,9 @@ program main
   real(real64), parameter :: nu=1.,final_time=0.6_real64,tolerance=1.E-3_real64,safety_factor=0.1_real64
   real(real64) :: time=0.,dt,dx
   integer, parameter :: nodes=16
-  procedure(initial_condition), pointer :: initial_u=>ten_sin
+  procedure(initial_condition), pointer :: initial_u=>null()
+
+  initial_u => ten_sin
 
 #ifdef TAU
   call TAU_PROFILE_SET_NODE(this_image()-1) ! Start TAU (Cray or GNU compiler)