gdb: Fix skip of `\r` before `\n` in source output

gdb: Fix skip of `\r` before `\n` in source output

In this commit:

  commit 62f29fda90cf1d5a1899f57ef78452471c707fd6
  Date:   Tue Oct 9 22:21:05 2018 -0600

      Highlight source code using GNU Source Highlight

A bug was introduced such that when displaying source code from a file
with lines `\r\n` GDB would print `^M` at the end of each line.

This caused a regression on the test gdb.fortran/nested-funcs.exp,
which happens to have `\r\n` line endings.

gdb/ChangeLog:

	* source.c (print_source_lines_base): Fix skip of '\r' if next
	character is '\n'.