[Bug preprocessor/105005] New: Misleading error message at #line directive with no arguments

[burda.ludek at seznam dot cz]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105005

            Bug ID: 105005
           Summary: Misleading error message at #line directive with no
                    arguments
           Product: gcc
           Version: 7.5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: preprocessor
          Assignee: unassigned at gcc dot gnu.org
          Reporter: burda.ludek at seznam dot cz
  Target Milestone: ---

Preprocessor gives a
"error: unexpected end of file after #line"
error message when no arguments are provided for #line directive, although
there is no actual EOF after the directive.

exemplar code:

```example.c
#line
#warning "some warning, not an EOF"
```
console output after "gcc -E example.c -o example" command

```
example.c:1:6: error: unexpected end of file after #line
 #line
      ^
example.c:2:2: warning: #warning "some warning, not an EOF" [-Wcpp]
 #warning "some warning, not an EOF"
  ^~~~~~~
```

This had been tested on 
gcc version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)

Configured with: ../src/configure -v --with-pkgversion='Ubuntu
7.5.0-3ubuntu1~18.04' --with-bugurl=file:///usr/share/doc/gcc-7/README.Bugs
--enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++ --prefix=/usr
--with-gcc-major-version-only --program-suffix=-7
--program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id
--libexecdir=/usr/lib --without-included-gettext --enable-threads=posix
--libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu
--enable-libstdcxx-debug --enable-libstdcxx-time=yes
--with-default-libstdcxx-abi=new --enable-gnu-unique-object
--disable-vtable-verify --enable-libmpx --enable-plugin --enable-default-pie
--with-system-zlib --with-target-system-zlib --enable-objc-gc=auto
--enable-multiarch --disable-werror --with-arch-32=i686 --with-abi=m64
--with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic
--enable-offload-targets=nvptx-none --without-cuda-driver
--enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu
--target=x86_64-linux-gnu

I managed to find the invalid piece of code causing this issue, it can be found
even in the newest gcc github version https://github.com/gcc-mirror . I'll
create a pull request with a fix of the bug rightaway I submit this bug report
so I can link this bug report there. I never participated in contributing
before, I'd be grateful for any kind of feedback. I followed the instructions
from there https://gcc.gnu.org/bugs/#need and I hope I didn't ommit any
necessary information in this bug report, otherwise please let me know and I
can provide such.