[Patch, Fortran] PR20888 using NULL() pointer as operand

[Tobias Burnus]

:ADDPATCH fortran:

Reject the following invalid code:


   IF (TEST .EQ. NULL(TEST)) TEST=>NULL() ! { dg-error "Invalid context
for NULL
     1
Error: Invalid context for NULL() pointer at (1)
  IF (NULL(TEST) .EQ. TEST) TEST=>NULL() ! { dg-error "Invalid context
for NULL
     1
Error: Invalid context for NULL() pointer at (1)


Any suggestions for a better error message?

Ok for the trunk? Build & regression tested on x86_64-unknown-linux-gnu
with no failures but PR32432.

Tobias