gdb_server: fix clang static analyzer warning

gdb_server: fix clang static analyzer warning

Warning:	line 373, column 15
Assigned value is garbage or undefined

Most probably a false warning, building the hex string byte
per byte seems too complicated for static analyze.

Change malloc to calloc to silence the warning.

Change-Id: I746d43fa51abf05582ccf2680ed72dc557798a7a
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/5905
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins
Reviewed-by: Jonathan McDowell <noodles-openocd@earth.li>