build: avoid compilation warnings by using memcpy() instead of strncpy()

When compiling with GCC 10.1.0 and -O2 -Wall, the strncpy() call
in measured_copy() produced two stringop-truncation warnings.

This addresses https://savannah.gnu.org/bugs/?58861.

Signed-off-by: Michalis Kokologiannakis <michalis@mpi-sws.org>

src/utils.c

@@ -330,7 +330,7 @@ char *measured_copy(const char *string, size_t count)
 {
 	char *thecopy = charalloc(count + 1);
 
-	strncpy(thecopy, string, count);
+	memcpy(thecopy, string, count);
 	thecopy[count] = '\0';
 
 	return thecopy;