Zero-initialize comm pack buffers to avoid undefined padding

When packing integer(FP_SIZE) arrays into real(FP_COMM) buffers, an
odd number of int32 values leaves padding bytes undefined in the last
double. This caused roundtrip test failures on gcc-13 where the
padding differed between pack cycles. Also fixes Intel 2025 segfault
in expand by using array(:) = transfer(..., size=size(array)) to
prevent allocatable reallocation.

Author: perazz

src/fitpack_core.F90

@@ -18360,6 +18360,8 @@ pure subroutine FP_REAL_COMM_PACK_1D(array, buffer)
         integer(FP_SIZE) :: bnd(2), ndoubles
         integer(FP_SIZE), parameter :: header = 1
 
+        buffer(:) = 0.0_FP_COMM
+
         if (allocated(array)) then
             bnd = [lbound(array, 1, FP_SIZE), ubound(array, 1, FP_SIZE)]
         else
@@ -18382,6 +18384,8 @@ pure subroutine FP_REAL_COMM_PACK_2D(array, buffer)
         integer(FP_SIZE) :: bnd(2, 2), ndoubles
         integer(FP_SIZE), parameter :: header = 2
 
+        buffer(:) = 0.0_FP_COMM
+
         if (allocated(array)) then
             bnd(1, 1) = lbound(array, 1, FP_SIZE)
             bnd(2, 1) = ubound(array, 1, FP_SIZE)
@@ -18407,6 +18411,8 @@ pure subroutine FP_REAL_COMM_PACK_3D(array, buffer)
         integer(FP_SIZE) :: bnd(2, 3), ndoubles
         integer(FP_SIZE), parameter :: header = 3
 
+        buffer(:) = 0.0_FP_COMM
+
         if (allocated(array)) then
             bnd(1, 1) = lbound(array, 1, FP_SIZE)
             bnd(2, 1) = ubound(array, 1, FP_SIZE)
@@ -18434,6 +18440,9 @@ pure subroutine FP_SIZE_COMM_PACK_1D(array, buffer)
         integer(FP_SIZE) :: bnd(2), ndoubles
         integer(FP_SIZE), parameter :: header = 1
 
+        ! Zero buffer to avoid undefined padding when int32 count is odd
+        buffer(:) = 0.0_FP_COMM
+
         if (allocated(array)) then
             bnd = [lbound(array, 1, FP_SIZE), ubound(array, 1, FP_SIZE)]
         else