Fix warnings on asprintf return value checking

[robtaylor]

eg:

./../lefdef/src/def/def/def.y:5098:24: warning: ignoring return value of ‘int asprintf(char**, const char*, ...)’ declared with attribute ‘warn_unused_result’ [-Wunused-result]
 5098 |               asprintf (&defData->defMsg,
      |               ~~~~~~~~~^~~~~~~~~~~~~~~~~~
 5099 |                  "The STYLES statement is available in version 5.6 and later.\nHowever, your DEF file is defined with version %g", defData->VersionNum);
      |                  ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~