Merge pull request #7299 from ARMmbed/g-fix-mbr-invalid

MBR: Fix issue with MBR init asserting on bad partition size

Author: 0xc0170

features/filesystem/bd/MBRBlockDevice.cpp

@@ -241,7 +241,9 @@ int MBRBlockDevice::init()
     _size   = fromle32(table->entries[_part-1].lba_size)   * sector;
 
     // Check that block addresses are valid
-    MBED_ASSERT(_bd->is_valid_erase(_offset, _size));
+    if (!_bd->is_valid_erase(_offset, _size)) {
+        return BD_ERROR_INVALID_PARTITION;
+    }
 
     delete[] buffer;
     return 0;