dahdi-base: fix potential underflow of unsigned type

Compile fails on newer kernels due to better fortification of memcpy
calls.

In function 'strncat',
    inlined from 'dahdi_ioctl_get_version' at dahdi-linux-3.4.0/drivers/dahdi/dahdi-base.c:5405:3:
./include/linux/fortify-string.h:114:33: error: '__builtin_memcpy' accessing 4294967295 bytes at offsets [80, 238] and 0 overlaps 6442450943 bytes at offset -2147483648 [-Werror=restrict]
  114 | #define __underlying_memcpy     __builtin_memcpy
      |                                 ^
./include/linux/fortify-string.h:457:9: note: in expansion of macro '__underlying_memcpy'
  457 |         __underlying_memcpy(p + p_len, q, copy_len);
      |         ^~~~~~~~~~~~~~~~~~~

Fix this by avoiding a potential underflow of unsigned type size_t.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>

Author: dangowrt

drivers/dahdi/dahdi-base.c

@@ -5380,7 +5380,7 @@ static int dahdi_ioctl_get_version(unsigned long data)
 {
 	struct dahdi_versioninfo vi;
 	struct ecfactory *cur;
-	size_t space = sizeof(vi.echo_canceller) - 1;
+	size_t space = sizeof(vi.echo_canceller) - 1, ec_name_len;
 	bool have_hwec = dahdi_any_hwec_available();
 
 	memset(&vi, 0, sizeof(vi));
@@ -5404,9 +5404,10 @@ static int dahdi_ioctl_get_version(unsigned long data)
 		}
 		strncat(vi.echo_canceller + strlen(vi.echo_canceller),
 			ec_name, space);
-		space -= strlen(ec_name);
-		if (space < 1)
+		ec_name_len = strlen(ec_name);
+		if (ec_name_len > space + 1)
 			break;
+		space -= ec_name_len;
 		if (cur->list.next && (cur->list.next != &ecfactory_list)) {
 			strncat(vi.echo_canceller + strlen(vi.echo_canceller),
 				", ", space);