gdb.mi/mi-pending.c: Return NULL instead of nothing in thread function

Using gcc 5.2 (maybe other versions as well), building mi-pending.c gives
these warnings:

	./gdb.mi/mi-pending.c: In function ‘thread_func’:
	./gdb.mi/mi-pending.c:34:5: warning: ‘return’ with no value, in function returning non-void
	     return;
	     ^
	./gdb.mi/mi-pending.c:38:5: warning: ‘return’ with no value, in function returning non-void
	     return;
	     ^
gdb_compile_pthreads assumes that the build was successful only if there
is no output. These warnings therefore make gdb_compile_pthreads think
that the build failed, and the test doesn't run.

The easy fix is to replace the "return" with "return NULL".  I am
pushing this as obvious.

gdb/testsuite/ChangeLog:

	* gdb.mi/mi-pending.c (thread_func): Replace return with return
	NULL.