index: Fix volk_get_index

This function results in an infinite loop on Debian 11 for some impls.
This is a first step to fix it.

Fix gnuradio#516

Signed-off-by: Johannes Demel <demel@uni-bremen.de>

lib/volk_rank_archs.c

@@ -34,13 +34,16 @@ int volk_get_index(const char* impl_names[], // list of implementations by name
 {
     unsigned int i;
     for (i = 0; i < n_impls; i++) {
-        if (!strncmp(impl_names[i], impl_name, 20)) {
+        if (!strncmp(impl_names[i], impl_name, 42)) {
             return i;
         }
     }
     // TODO return -1;
     // something terrible should happen here
     fprintf(stderr, "Volk warning: no arch found, returning generic impl\n");
+    if (strncmp(impl_name, "generic", 20)) {
+        return -1;
+    }
     return volk_get_index(impl_names, n_impls, "generic"); // but we'll fake it for now
 }