Fixes this GCC warning:

libsvmread.c: In function 'mexFunction':
libsvmread.c:202:21: warning: the comparison will always evaluate as 'false' for the address of 'filename' will never be NULL [-Waddress]
  202 |         if(filename == NULL)
      |                     ^~
libsvmread.c:191:14: note: 'filename' declared here
  191 |         char filename[256];
      |              ^~~~~~~~

diff -up matlab/libsvmread.c.orig matlab/libsvmread.c
--- matlab/libsvmread.c.orig	2022-08-10 08:06:25.000000000 -0600
+++ matlab/libsvmread.c	2022-08-12 09:17:03.079895620 -0600
@@ -199,7 +199,7 @@ void mexFunction( int nlhs, mxArray *plh
 
 	mxGetString(prhs[0], filename, mxGetN(prhs[0]) + 1);
 
-	if(filename == NULL)
+	if(filename[0] == '\0')
 	{
 		mexPrintf("Error: filename is NULL\n");
 		return;