Fix: may be used uninitialized trace_name variable

Fix: may be used uninitialized trace_name variable

This fixes the following Clang warning:
  dmesg.c:263:7: error: variable 'trace_name' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
                if (strcmp(basename, G_DIR_SEPARATOR_S) != 0 &&
                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  dmesg.c:269:6: note: uninitialized use occurs here
          if (trace_name) {
              ^~~~~~~~~~
  dmesg.c:263:3: note: remove the 'if' if its condition is always true
                  if (strcmp(basename, G_DIR_SEPARATOR_S) != 0 &&
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  dmesg.c:263:7: error: variable 'trace_name' is used uninitialized whenever '&&' condition is false [-Werror,-Wsometimes-uninitialized]
                  if (strcmp(basename, G_DIR_SEPARATOR_S) != 0 &&
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  dmesg.c:269:6: note: uninitialized use occurs here
          if (trace_name) {
              ^~~~~~~~~~
  dmesg.c:263:7: note: remove the '&&' if its condition is always true
                  if (strcmp(basename, G_DIR_SEPARATOR_S) != 0 &&
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  dmesg.c:248:24: note: initialize the variable 'trace_name' to silence this warning
          const char *trace_name;

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>