spirv-val: fix another C4146 warning (KhronosGroup#6289)

Required to pass -Werror for DXC+MSVC

Author: Keenuts

source/val/validate_conversion.cpp

@@ -14,6 +14,8 @@
 
 // Validates correctness of conversion instructions.
 
+#include <climits>
+
 #include "source/opcode.h"
 #include "source/spirv_constant.h"
 #include "source/spirv_target_env.h"
@@ -715,7 +717,7 @@ spv_result_t ConversionPass(ValidationState_t& _, const Instruction* inst) {
       unsigned res_arr_len_id = result_type_inst->GetOperandAs<unsigned>(2u);
 
       // Are the input and result element types compatible?
-      unsigned src_arr_len = -1u, res_arr_len = -1u;
+      unsigned src_arr_len = UINT_MAX, res_arr_len = UINT_MAX;
       bool src_arr_len_status =
           _.GetConstantValueAs<unsigned>(src_arr_len_id, src_arr_len);
       bool res_arr_len_status =