Fix a potential double-free in ecpg.

Author: Michael Meskes

src/interfaces/ecpg/preproc/pgc.l

@@ -541,6 +541,7 @@ cppline			{space}*#([^i][A-Za-z]*|{if}|{ifdef}|{ifndef}|{import})(.*\\{space})*.
 				{
 					addlit(yytext, yyleng);
 					free(dolqstart);
+					dolqstart = NULL;
 					BEGIN(SQL);
 					base_yylval.str = mm_strdup(literalbuf);
 					return DOLCONST;