Tweak previous patch to ensure edata->filename always gets initialized.

On a platform that isn't supplying __FILE__, previous coding would either
crash or give a stale result for the filename string.  Not sure how likely
that is, but the original code catered for it, so let's keep doing so.

src/backend/utils/error/elog.c

@@ -348,8 +348,10 @@ errstart(int elevel, const char *filename, int lineno,
 
 		/* keep only base name, useful especially for vpath builds */
 		slash = strrchr(filename, '/');
-		edata->filename = slash ? slash + 1 : filename;
+		if (slash)
+			filename = slash + 1;
 	}
+	edata->filename = filename;
 	edata->lineno = lineno;
 	edata->funcname = funcname;
 	/* the default text domain is the backend's */
@@ -1154,8 +1156,10 @@ elog_start(const char *filename, int lineno, const char *funcname)
 
 		/* keep only base name, useful especially for vpath builds */
 		slash = strrchr(filename, '/');
-		edata->filename = slash ? slash + 1 : filename;
+		if (slash)
+			filename = slash + 1;
 	}
+	edata->filename = filename;
 	edata->lineno = lineno;
 	edata->funcname = funcname;
 	/* errno is saved now so that error parameter eval can't change it */