gallivm: Add constructor for raw_debug_ostream.

Fixes uninitialized scalar field defect reported by Coverity.

Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: José Fonseca <jfonseca@vmware.com>

src/gallium/auxiliary/gallivm/lp_bld_debug.cpp

@@ -81,8 +81,12 @@ lp_check_alignment(const void *ptr, unsigned alignment)
 class raw_debug_ostream :
    public llvm::raw_ostream
 {
+private:
    uint64_t pos;
 
+public:
+   raw_debug_ostream() : pos(0) { }
+
    void write_impl(const char *Ptr, size_t Size);
 
 #if HAVE_LLVM >= 0x207