lib: strto: Stop detection when invalid char is used

This issue has been found when mtd partition are specified. Autodetection
code should stop when the first invalid char is found.

Here is the example of commands:
setenv mtdids nand0=memory-controller@e000e000
setenv mtdparts "mtdparts=nand0:4m(boot),4m(env),64m(kernel),96m(rootfs)"
mtd list

Before:
Zynq> mtd list
List of MTD devices:
* nand0
  - type: NAND flash
  - block size: 0x20000 bytes
  - min I/O: 0x800 bytes
  - OOB size: 64 bytes
  - OOB available: 16 bytes
  - ECC strength: 1 bits
  - ECC step size: 2048 bytes
  - bitflip threshold: 1 bits
  - 0x000000000000-0x000010000000 : "nand0"
	  - 0x000000000000-0x000000400000 : "boot"
	  - 0x000000400000-0x000000800000 : "env"
	  - 0x000000800000-0x000006c00000 : "kernel"
	  - 0x000006c00000-0x000010000000 : "rootfs"

Where it is visible that kernel partition has 100m instead of 64m

After:
Zynq> mtd list
* nand0
  - type: NAND flash
  - block size: 0x20000 bytes
  - min I/O: 0x800 bytes
  - OOB size: 64 bytes
  - OOB available: 16 bytes
  - ECC strength: 1 bits
  - ECC step size: 2048 bytes
  - bitflip threshold: 1 bits
  - 0x000000000000-0x000010000000 : "nand0"
	  - 0x000000000000-0x000000400000 : "boot"
	  - 0x000000400000-0x000000800000 : "env"
	  - 0x000000800000-0x000004800000 : "kernel"
	  - 0x000004800000-0x00000a800000 : "rootfs"

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Fixes: 0486497e ("lib: Improve _parse_integer_fixup_radix base 16 detection")
Tested-by: Heiko Schocher <hs@denx.de>
Tested-by: Pali Rohár <pali@kernel.org>