wordrec: Fix compiler warning (-Wstringop-truncation) (#1398)

gcc warning:
wordrec/language_model.cpp:959:16: warning:
 ‘char* strncpy(char*, const char*, size_t)’ output truncated before
 terminating nul copying as many bytes from a string as its length
 [-Wstringop-truncation]

memcpy could also be a little bit faster than strncpy.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

wordrec/language_model.cpp

@@ -953,10 +953,10 @@ float LanguageModel::ComputeNgramCost(const char *unichar,
     // unless use_only_first_uft8_step is true.
     if (unichar_ptr < unichar_end) {
       if (modified_context == NULL) {
-        int context_len = strlen(context);
+        size_t context_len = strlen(context);
         modified_context =
           new char[context_len + strlen(unichar_ptr) + step + 1];
-        strncpy(modified_context, context, context_len);
+        memcpy(modified_context, context, context_len);
         modified_context_end = modified_context + context_len;
         context_ptr = modified_context;
       }