Fix NaN check reading wrong index in MPI unpack

The NaN diagnostic check used q_comm(i)%sf(j, k, l) but the value
was unpacked into q_comm(i)%sf(j + unpack_offset, k, l). This meant
the check was reading a stale or unrelated cell instead of the just-
received value.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: sbryngelson

src/common/m_mpi_common.fpp

@@ -936,7 +936,7 @@ contains
                                         (j + buff_size*((k + 1) + (n + 1)*l))
                                     q_comm(i)%sf(j + unpack_offset, k, l) = real(buff_recv(r), kind=stp)
 #if defined(__INTEL_COMPILER)
-                                    if (ieee_is_nan(q_comm(i)%sf(j, k, l))) then
+                                    if (ieee_is_nan(q_comm(i)%sf(j + unpack_offset, k, l))) then
                                         print *, "Error", j, k, l, i
                                         error stop "NaN(s) in recv"
                                     end if
@@ -991,7 +991,7 @@ contains
                                          ((k + buff_size) + buff_size*l))
                                     q_comm(i)%sf(j, k + unpack_offset, l) = real(buff_recv(r), kind=stp)
 #if defined(__INTEL_COMPILER)
-                                    if (ieee_is_nan(q_comm(i)%sf(j, k, l))) then
+                                    if (ieee_is_nan(q_comm(i)%sf(j, k + unpack_offset, l))) then
                                         print *, "Error", j, k, l, i
                                         error stop "NaN(s) in recv"
                                     end if
@@ -1050,7 +1050,7 @@ contains
                                           (l + buff_size)))
                                     q_comm(i)%sf(j, k, l + unpack_offset) = real(buff_recv(r), kind=stp)
 #if defined(__INTEL_COMPILER)
-                                    if (ieee_is_nan(q_comm(i)%sf(j, k, l))) then
+                                    if (ieee_is_nan(q_comm(i)%sf(j, k, l + unpack_offset))) then
                                         print *, "Error", j, k, l, i
                                         error stop "NaN(s) in recv"
                                     end if