Fix ValueError/IndexError crash in SafeLoader for malformed numeric scalars (#898)

[Nicolas0315]

Summary

Fix uncaught ValueError/IndexError in SafeLoader when parsing malformed numeric scalars like 0x_, !!float +_, and !!float 1::3.

Root Cause

construct_yaml_int and construct_yaml_float strip underscores from the scalar value before parsing, but don't handle the case where stripping leaves an empty or invalid string:

Input	After replace('_', '')	Failure
0x_	0x	int('', 16) → ValueError
0b_	0b	int('', 2) → ValueError
!!float +_	+ → ``(empty after sign strip)	value[0] → IndexError
!!float 1::3	1::3	float('') (empty split part) → ValueError

Fix

Wrap the parsing logic in try/except to convert ValueError and IndexError into ConstructorError, which is the expected exception type for invalid YAML values. This matches the behavior of other constructors in PyYAML (e.g. construct_yaml_binary).

Testing

import yaml

# All now raise ConstructorError instead of ValueError/IndexError:
yaml.safe_load('0x_')        # ConstructorError ✓
yaml.safe_load('0b_')        # ConstructorError ✓  
yaml.safe_load('!!float +_') # ConstructorError ✓
yaml.safe_load('!!float 1::3') # ConstructorError ✓

# Normal values still work:
yaml.safe_load('42')        # 42 ✓
yaml.safe_load('0x1F')      # 31 ✓
yaml.safe_load('3.14')      # 3.14 ✓

Fixes #898