[Patch, fortran] PR20880 - Failure to detect procedures encompassing own interface

[François-Xavier Coudert]

Take care that there is no reason for the backtrace of a sefault to
actually point to the first corruption of memory. It is more often
accurate to report the backtrace while running the programming with
ElectricFence.

> Backtrace:
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x0000000000459fea in gfc_add_explicit_interface (sym=0xdf78a0,
> source=IFSRC_DECL, formal=0xdee3c0, where=0xcf6970)
>     at /home/tob/projects/gcc/gcc/fortran/symbol.c:1207
> 1207      if (current_interface.ns
> (gdb) bt
> #0  0x0000000000459fea in gfc_add_explicit_interface (sym=0xdf78a0,
> source=IFSRC_DECL, formal=0xdee3c0, where=0xcf6970)
>     at /home/tob/projects/gcc/gcc/fortran/symbol.c:1207
> #1  0x0000000000413b2f in gfc_match_formal_arglist (progname=0xdf78a0,
> st_flag=1, null_flag=14607296)
>     at /home/tob/projects/gcc/gcc/fortran/decl.c:2693
> #2  0x0000000000435c06 in gfc_match_st_function () at
> /home/tob/projects/gcc/gcc/fortran/match.c:2937
> #3  0x0000000000441083 in match_word (str=0xdf78d8 "", subr=0x435b90
> <gfc_match_st_function>, old_locus=0x7fff4b554a60)
>     at /home/tob/projects/gcc/gcc/fortran/parse.c:65