[patch, fortran] PR77972 ICE on broken character continuation with -Wall etc.

[Jerry DeLisle]

This patch is straight forward. We were sending bogus locus info to the 
diagnostics machinery and catch an assert in error,c.

The patch avoids doing this.
Regression tested on x86-64-linux.

OK for trunk?

Regards,

Jerry

2016-10-13  Jerry DeLisle  <jvdeli...@gcc.gnu.org>

        * scanner.c (gfc_next_char_literal): If nextc is null do not
        decrement the pointer and call the diagnostics.

diff --git a/gcc/fortran/scanner.c b/gcc/fortran/scanner.c
index be9c5091..5e355359 100644
--- a/gcc/fortran/scanner.c
+++ b/gcc/fortran/scanner.c
@@ -1414,10 +1414,9 @@ restart:

       if (c != '&')
        {
-         if (in_string)
+         if (in_string && gfc_current_locus.nextc)
            {
-             if (gfc_current_locus.nextc)
-               gfc_current_locus.nextc--;
+             gfc_current_locus.nextc--;
              if (warn_ampersand && in_string == INSTRING_WARN)
                gfc_warning (OPT_Wampersand,
                             "Missing %<&%> in continued character "