text: introduce text_bytes_alloc0 utility function

Heap allocates a zero terminated string of the given range.
Freeing is the caller's responsibility.

Checks for unsigned integer overflow i.e. passing SIZE_MAX
as len will always fail because there is no room for the
terminating NUL byte. This is important as EPOS is defined
to be SIZE_MAX.

text.c

@@ -21,6 +21,7 @@
 #include <fcntl.h>
 #include <errno.h>
 #include <wchar.h>
+#include <stdint.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <sys/mman.h>
@@ -1441,6 +1442,17 @@ size_t text_bytes_get(Text *txt, size_t pos, size_t len, char *buf) {
 	return len - rem;
 }
 
+char *text_bytes_alloc0(Text *txt, size_t pos, size_t len) {
+	if (len == SIZE_MAX)
+		return NULL;
+	char *buf = malloc(len+1);
+	if (!buf)
+		return NULL;
+	len = text_bytes_get(txt, pos, len, buf);
+	buf[len] = '\0';
+	return buf;
+}
+
 size_t text_size(Text *txt) {
 	return txt->size;
 }

text.h

@@ -72,6 +72,9 @@ bool text_byte_get(Text*, size_t pos, char *buf);
  * indicates how many bytes were copied into `buf'. WARNING buf will not be
  * NUL terminated. */
 size_t text_bytes_get(Text*, size_t pos, size_t len, char *buf);
+/* allocate a NUL terminated buffer and fill at most `len' bytes
+ * starting at `pos'. Freeing is the caller's responsibility! */
+char *text_bytes_alloc0(Text*, size_t pos, size_t len);
 
 Iterator text_iterator_get(Text*, size_t pos);
 bool text_iterator_valid(const Iterator*);