Don't allocate the string until error conditions have been checked

Fixes: a00cfed0 ("Hexagon (disas) disassembler")
Eliminate Coverity CID 1460121 (Resource leak)

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daud? <f4bug@amsat.org>
Signed-off-by: Taylor Simpson <tsimpson@quicinc.com>