gallium/aux: Fix Wmaybe-uninitialized warning
I got this warning with mingw:
../src/gallium/auxiliary/draw/draw_llvm.c: In function 'draw_llvm_generate':
../src/gallium/auxiliary/draw/draw_llvm.c:1663:25: warning: 'sum' may be used uninitialized in this function [-Wmaybe-uninitialized]
1663 | sum = lp_build_fmuladd(builder, planes,
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1664 | (LLVMValueRef[]){cv_x, cv_y, cv_z, cv_w}[i], sum);
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../src/gallium/auxiliary/draw/draw_llvm.c:1511:44: note: 'sum' was declared here
1511 | LLVMValueRef plane1, planes, plane_ptr, sum;
| ^~~