compilation failed with -Werror=maybe-uninitialized

The compilation failed for devmode
--enable DONT_OPTIMIZE
--enable BETTER_BACKTRACES
--enable DO_CRASH
--enable TEST_FRAMEWORK

res_pjsip/pjsip_configuration.c: In function dtls_handler:
res_pjsip/pjsip_configuration.c:974:20: error:
back may be used uninitialized in this function [-Werror=maybe-uninitialized]
int size = strlen(front);
           ^
cc1: all warnings being treated as errors

Change-Id: I7f082ead0312792a577ec7c73015ba64dabca580