swr: avoid using exceptions for expected condition handling

swr: avoid using exceptions for expected condition handling

I was getting a weird segfault from GCC 4.9.3:

 0x00007ffff54f27aa in strlen () from /lib64/libc.so.6
 (gdb) bt
 #0  0x00007ffff54f27aa in strlen () from /lib64/libc.so.6
 #1  0x00007ffff4f128e5 in get_cie_encoding (cie=cie@entry=0x7ffff6e09813)
     at /gcc-4.9.3/libgcc/unwind-dw2-fde.c:272
 #2  0x00007ffff4f1318e in classify_object_over_fdes (ob=ob@entry=0xd7bb90, this_fde=0x7ffff7f11010)
     at /gcc-4.9.3/libgcc/unwind-dw2-fde.c:628
 #3  0x00007ffff4f135ba in init_object (ob=0xd7bb90)
     at /gcc-4.9.3/libgcc/unwind-dw2-fde.c:749
 #4  search_object (ob=ob@entry=0xd7bb90, pc=pc@entry=0x7ffff4f11f4d <_Unwind_RaiseException+61>)
     at /gcc-4.9.3/libgcc/unwind-dw2-fde.c:961
 #5  0x00007ffff4f13e62 in _Unwind_Find_registered_FDE (bases=0x7fffffffd358, pc=0x7ffff4f11f4d <_Unwind_RaiseException+61>)
     at /gcc-4.9.3/libgcc/unwind-dw2-fde.c:1025
 #6  _Unwind_Find_FDE (pc=0x7ffff4f11f4d <_Unwind_RaiseException+61>, bases=bases@entry=0x7fffffffd358)
     at /gcc-4.9.3/libgcc/unwind-dw2-fde-dip.c:450
 #7  0x00007ffff4f11197 in uw_frame_state_for (context=context@entry=0x7fffffffd2b0, fs=fs@entry=0x7fffffffd100)
     at /gcc-4.9.3/libgcc/unwind-dw2.c:1245
 #8  0x00007ffff4f11b15 in uw_init_context_1 (context=context@entry=0x7fffffffd2b0, outer_cfa=outer_cfa@entry=0x7fffffffd660, outer_ra=0x7ffff518d23b <__cxa_throw+91>)
     at /gcc-4.9.3/libgcc/unwind-dw2.c:1566
 #9  0x00007ffff4f11f4e in _Unwind_RaiseException (exc=0xd7c250)
     at /gcc-4.9.3/libgcc/unwind.inc:88
 #10 0x00007ffff518d23b in __cxa_throw () from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/libstdc++.so.6
 #11 0x00007ffff51ed556 in std::__throw_out_of_range(char const*) ()
    from /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/libstdc++.so.6
 #12 0x00007fffea778be0 in std::map<pipe_format, SWR_FORMAT, std::less<pipe_format>, std::allocator<std::pair<pipe_format const, SWR_FORMAT> > >::at (
     this=0x7fffebeb4c40 <mesa_to_swr_format(pipe_format)::mesa2swr>,
     __k=@0x7fffffffd73c: PIPE_FORMAT_RGTC1_UNORM)
     at /usr/lib/gcc/x86_64-pc-linux-gnu/4.9.3/include/g++-v4/bits/stl_map.h:549
 #13 0x00007fffea776aee in mesa_to_swr_format (format=PIPE_FORMAT_RGTC1_UNORM) at swr_screen.cpp:597

We can just void this whole issue by not using exceptions in the
first place.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Bruce Cherniak <bruce.cherniak@intel.com>