fstrim: initialize path variable

Reading the code this initialization is not necessary, so the change is
mostly about silencing a compiler warning and ensuring the program keeps
on working if there is a future bug that could make uninitialized 'path'
reachable to use.

sys-utils/fstrim.c:247: warning: 'path' may be used uninitialized in this
function

Signed-off-by: Sami Kerola <kerolasa@iki.fi>

sys-utils/fstrim.c

@@ -244,7 +244,7 @@ static void __attribute__((__noreturn__)) usage(FILE *out)
 
 int main(int argc, char **argv)
 {
-	char *path;
+	char *path = NULL;
 	int c, rc, verbose = 0, all = 0;
 	struct fstrim_range range;