st/clover: Pass target instead of target.begin() to std::string()

st/clover: Pass target instead of target.begin() to std::string()

Fixes reading beyond allocated memory:

==1936== Invalid read of size 1
==1936==    at 0x4C2C1B4: strlen (vg_replace_strmem.c:412)
==1936==    by 0x9E00C30: std::basic_string<char, std::char_traits<char>, std::allocator<char> >::basic_string(char const*, std::allocator<char> const&) (in /usr/lib/x86_64-linux-gnu/libstdc++.so.6.0.20)
==1936==    by 0x5B44FAE: clover::compile_program_llvm(clover::compat::string const&, clover::compat::vector<clover::compat::pair<clover::compat::string, clover::compat::string> > const&, pipe_shader_ir, clover::compat::string const&, clover::compat::string const&, clover::compat::string&) (invocation.cpp:698)
==1936==    by 0x5B39A20: clover::program::build(clover::ref_vector<clover::device> const&, char const*, clover::compat::vector<clover::compat::pair<clover::compat::string, clover::compat::string> > const&) (program.cpp:63)
==1936==    by 0x5B20152: clBuildProgram (program.cpp:182)
==1936==    by 0x400F41: main (hello_world.c:109)
==1936==  Address 0x56fee1f is 0 bytes after a block of size 15 alloc'd
==1936==    at 0x4C28C20: malloc (vg_replace_malloc.c:296)
==1936==    by 0x5B398F0: alloc (compat.hpp:59)
==1936==    by 0x5B398F0: vector<std::basic_string<char> > (compat.hpp:98)
==1936==    by 0x5B398F0: string<std::basic_string<char> > (compat.hpp:327)
==1936==    by 0x5B398F0: clover::program::build(clover::ref_vector<clover::device> const&, char const*, clover::compat::vector<clover::compat::pair<clover::compat::string, clover::compat::string> > const&) (program.cpp:63)
==1936==    by 0x5B20152: clBuildProgram (program.cpp:182)
==1936==    by 0x400F41: main (hello_world.c:109)

Reviewed-by: Francisco Jerez <currojerez@riseup.net>