filter_libc: remove the line holding the futex syscall error entirely

The current filter might leave empty lines behind.
This is caused by the fact that glibc used to not include a newline
in the message. But since glibc 2.29 it does.

https://bugs.kde.org/show_bug.cgi?id=405205
Reported-by: Stefan Maksimovic <stefan.maksimovic@rt-rk.com>

NEWS

@@ -129,6 +129,7 @@ where XXXXXX is the bug number as listed below.
 404888  autotools cleanup series
 405079  unhandled ppc64le-linux syscall: 131 (quotactl)
 405182  Valgrind fails to build with Clang
+405205  filter_libc: remove the line holding the futex syscall error entirely
 405356  PPC64, xvcvsxdsp, xvcvuxdsp are supposed to write the 32-bit result to
         the upper and lower 32-bits of the 64-bit result
 405362  PPC64, vmsummbm instruction doesn't handle overflow case correctly

tests/filter_libc

@@ -37,6 +37,8 @@ while (<>)
     s/(at.*)operator delete\[\]\(void\*\)/$1...operator delete[].../;
 
     # Some glibc versions complain about unexpected futex syscall errors.
+    # With or without newline (see sourceware PR20271).
+    next if /^The futex facility returned an unexpected error code.$/;
     s/The futex facility returned an unexpected error code.//;
 
     print;