emul_sbs_gauge: Don't set bad val in i2c msg buf

The SBS Gauge emulator would set the I2C buffer message with a value from
the emulator's custom read function even on a read error instead of just
returning. This behavior may be improperly relied upon by future tests.

Return early on register read before setting the i2c message buffer value
in the emulator.

Signed-off-by: Aaron Massey <aaronmassey@google.com>

subsys/emul/i2c/emul_sbs_gauge.c

@@ -105,6 +105,10 @@ static int sbs_gauge_emul_transfer_i2c(const struct emul *target, struct i2c_msg
 			switch (msgs->len - 1) {
 			case 1:
 				rc = reg_read(target, reg, &val);
+				if (rc) {
+					/* Return before writing bad value to message buffer */
+					return rc;
+				}
 				msgs->buf[0] = val;
 				break;
 			default: