add sp_hex_iszero for catching edge case of negative zero value

[JacobBarthelmeh]

Fix for oss-fuzz report when handling a sp_int that has 'used' greater than 0 but has a magnitude of 0.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #10655

Scan targets checked: wolfcrypt-bugs, wolfcrypt-src

No new issues found in the changed files. ✅

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[github-actions]

MemBrowse Memory Report

gcc-arm-cortex-m0plus

• FLASH: .text +32 B (+0.1%, 63,507 B / 262,144 B, total: 24% used)

gcc-arm-cortex-m3

• FLASH: .text +32 B (+0.0%, 121,285 B / 262,144 B, total: 46% used)

gcc-arm-cortex-m4-baremetal

• FLASH: .text +64 B (+0.1%, 66,123 B / 262,144 B, total: 25% used)

gcc-arm-cortex-m4-openssl-compat

• FLASH: .text +64 B (+0.0%, 766,828 B / 1,048,576 B, total: 73% used)

gcc-arm-cortex-m4-pq

• FLASH: .text +64 B (+0.0%, 277,592 B / 1,048,576 B, total: 26% used)

gcc-arm-cortex-m4-rsa-only

• FLASH: .text +64 B (+0.0%, 323,000 B / 1,048,576 B, total: 31% used)

gcc-arm-cortex-m4-tls12

• FLASH: .text +64 B (+0.1%, 122,061 B / 262,144 B, total: 47% used)

gcc-arm-cortex-m4-tls13

• FLASH: .text +64 B (+0.0%, 234,528 B / 262,144 B, total: 89% used)

gcc-arm-cortex-m7-pq

• FLASH: .text +64 B (+0.0%, 278,168 B / 1,048,576 B, total: 27% used)

linuxkm-pie

• Data: __patchable_function_entries +8 B (+0.0%, 24,272 B)
  No memory changes detected for:
• gcc-arm-cortex-m4
• gcc-arm-cortex-m4-crypto-only
• gcc-arm-cortex-m4-dtls13
• gcc-arm-cortex-m4-min-ecc
• gcc-arm-cortex-m4-pkcs7
• gcc-arm-cortex-m4-sp-math
• gcc-arm-cortex-m7
• gcc-arm-cortex-m7-tls13
• linuxkm-standard
• stm32-sim-stm32h753