Fix Ubuntu warning for asprintf in s-golay filter

Briscoe reported a warning about not using the return
value from a asprintf call on Ubuntu:

filter_sgolay.cpp:143:17: warning: ignoring return value
of ‘int asprintf(char**, const char*, ...)’ declared with attribute
‘warn_unused_result’ [-Wunused-result]
 143 |         asprintf(&buffer, "cannot build matrix with %lu rows and %lu columns\n",

libkoviz/filter_sgolay.cpp

@@ -140,10 +140,15 @@ float_mat::float_mat(const size_t rows,const size_t cols,const double defval)
     }
     if ((rows < 1) || (cols < 1)) {
         char* buffer;
-        asprintf(&buffer, "cannot build matrix with %lu rows and %lu columns\n",
-                rows, cols);
-        sgs_error(buffer);
-        free(buffer);
+        int ret = asprintf(&buffer,
+                          "cannot build matrix with %lu rows and %lu columns\n",
+                          rows, cols);
+        if ( ret > 0 ) {
+            sgs_error(buffer);
+            free(buffer);
+        } else {
+            sgs_error("koviz [error]: S-golay filter can't allocate memory!\n");
+        }
     }
 }