[Patch, fortran] Fix bogus warning with continued lines

[Jerry DeLisle]

Tobias Burnus wrote:
> :ADDPATCH fortran:
> 
> Fortran allows strings to continue by placing a & as last character in
> the line:
> 
> print *, "Hello&
>    & World"
> 
> gfortran supports as extension:
> 
> print *, "Hello&
>      World"
> 
> With either -Wall, -pedantic or -std=f95/f2003 the following error is shown:
> 
> Warning: Missing '&' in continued character constant at (1)
> 
> However, the same message is (wrongly) shown for
> 
> print *, "Hello"&
>     , " World"&
>     // "!"
> 
> The problem is that next_string_char read the " and needs to read the
> next character to check it is not another ". It erroneously calls 
> gfc_next_char_literal  with the option in_string = 1, which causes it to
> read up to the comma or slash.
> 
> The following patch corrects this and ensures also that the warning
> message is printed only once.
> 
> Thanks to Harald Anlauf for the report.
> 
> Bootstrapped and regression tested on x86-64/Linux.
> Ok for the trunk and 4.2? (As it is only a warning, I think we can omit
> 4.1.)

This is OK,

Jerry