Handle index register of a memory operation being the pseudo-register RIZ/EIZ

This should not adversely affect the rest of the logic as it is only ever entered in cases in which we would have asserted false before

Enables us to instrument x509

irdb-libs/libIRDB-core/src/operand_csx86.cpp

@@ -176,6 +176,9 @@ static uint32_t to_reg_number(const x86_reg &reg)
 		case X86_REG_R15D:
 		case X86_REG_R15W:
 			return 15;
+        case X86_REG_RIZ:
+        case X86_REG_EIZ:
+            return -1; // STARS_x86_R_none, RIZ/EIZ is a pseudo-register having constant value of 0
 		default: break;
 	}
 	assert(0);