sms-part-cdma: fix invalid memory read when parsing empty ascii text

Same fix also applied to latin encoded text as it also makes sense there.

  ==158856== Invalid read of size 1
  ==158856==    at 0x10B814: read_bits (mm-sms-part-cdma.c:257)
  ==158856==    by 0x10DB07: read_bearer_data_user_data (mm-sms-part-cdma.c:878)
  ==158856==    by 0x10DB07: read_bearer_data (mm-sms-part-cdma.c:990)
  ==158856==    by 0x10DB07: mm_sms_part_cdma_new_from_binary_pdu (mm-sms-part-cdma.c:1170)
  ==158856==    by 0x10DE54: mm_sms_part_cdma_new_from_pdu (mm-sms-part-cdma.c:333)
  ==158856==    by 0x10A916: common_test_invalid_part_from_hexpdu (test-sms-part-cdma.c:90)
  ==158856==    by 0x10A916: common_test_invalid_part_from_pdu (test-sms-part-cdma.c:104)
  ==158856==    by 0x4A0264D: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A023B4: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A023B4: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A023B4: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A023B4: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A02B1A: g_test_run_suite (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A02BBC: g_test_run (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x10A509: main (test-sms-part-cdma.c:595)
  ==158856==  Address 0x51a627b is 0 bytes after a block of size 11 alloc'd
  ==158856==    at 0x48455EF: calloc (vg_replace_malloc.c:1328)
  ==158856==    by 0x49DF6C0: g_malloc0 (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x48ABD24: mm_utils_hexstr2bin (mm-common-helpers.c:1884)
  ==158856==    by 0x10DE36: mm_sms_part_cdma_new_from_pdu (mm-sms-part-cdma.c:327)
  ==158856==    by 0x10A916: common_test_invalid_part_from_hexpdu (test-sms-part-cdma.c:90)
  ==158856==    by 0x10A916: common_test_invalid_part_from_pdu (test-sms-part-cdma.c:104)
  ==158856==    by 0x4A0264D: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A023B4: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A023B4: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A023B4: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A023B4: ??? (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A02B1A: g_test_run_suite (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==    by 0x4A02BBC: g_test_run (in /usr/lib/x86_64-linux-gnu/libglib-2.0.so.0.7400.2)
  ==158856==