vpr: utils: fix segfault in find_most_common_block_type

Recent logical/physical tiles split introduced the regression. If
there is no max_type found we should not call the logical_block_type
function, but rather return the nullptr maidenly (like we did before
the split).

Signed-off-by: Karol Gugala <[email protected]>

Author: kgugala

vpr/src/util/vpr_utils.cpp

@@ -779,6 +779,7 @@ t_logical_block_type_ptr find_most_common_block_type(const DeviceGrid& grid) {
 
     if (max_type == nullptr) {
         VTR_LOG_WARN("Unable to determine most common block type (perhaps the device grid was empty?)\n");
+        return nullptr;
     }
     return logical_block_type(max_type);
 }