Set segment's alloc_mode_text so we can free it (#7758)

LIke the dtata colums has S_>alloc_mode[col], the optional trailing text has alloc_mode_text but like all alloc)modes it defaults to EXTERNAL and needs to be set to INTERNAL when GMT is filling them in - otherwise we get a memory leaka.
a

src/gmt_io.c

@@ -7919,6 +7919,7 @@ int gmt_alloc_segment (struct GMT_CTRL *GMT, struct GMT_DATASEGMENT *S, uint64_t
 				GMT_Report (GMT->parent, GMT_MSG_ERROR, "gmt_alloc_segment: Unable to reallocate string array new length %" PRIu64 "\n", n_rows);
 				return 1;
 			}
+			SH->alloc_mode_text = GMT_ALLOC_INTERNALLY;
 		}
 		SH->n_alloc = n_rows;
 	}
@@ -7954,6 +7955,7 @@ void gmtlib_assign_segment (struct GMT_CTRL *GMT, unsigned int direction, struct
 				GMT->hidden.mem_txt = gmt_M_memory (GMT, GMT->hidden.mem_txt, n_rows, char *);	/* Trim back */
 			S->text = GMT->hidden.mem_txt;	/* Pass the pointer */
 			GMT->hidden.mem_txt = NULL;		/* Null this out to start over for next segment */
+			SH->alloc_mode_text = GMT_ALLOC_INTERNALLY;
 		}
 		GMT->hidden.mem_cols = 0;	/* Flag that we need to reallocate new temp arrays for next segment, if any */
 	}
@@ -7966,6 +7968,7 @@ void gmtlib_assign_segment (struct GMT_CTRL *GMT, unsigned int direction, struct
 		if (GMT->current.io.record_type[direction] & GMT_READ_TEXT) {
 			uint64_t row;
 			S->text = gmt_M_memory (GMT, S->text, n_rows, char *);
+			SH->alloc_mode_text = GMT_ALLOC_INTERNALLY;
 			for (row = 0; row < n_rows; row++) {
 				S->text[row] = GMT->hidden.mem_txt[row];
 				GMT->hidden.mem_txt[row] = NULL;