[libc] Fix missing UInt initialization (#74869)

Fix forward for #74862

Author: gchatelet

libc/src/__support/float_to_string.h

@@ -653,7 +653,7 @@ FloatToString<long double>::get_positive_block(int block_index) {
 
     // shift_amount = -(c0 - exponent) = c_0 + 16 * ceil(exponent/16) - exponent
 
-    cpp::UInt<MID_INT_SIZE> val;
+    cpp::UInt<MID_INT_SIZE> val(0);
 #ifdef LIBC_COPT_FLOAT_TO_STR_USE_MEGA_LONG_DOUBLE_TABLE
     // ------------------------------ TABLE MODE -------------------------------
     const int32_t SHIFT_CONST = TABLE_SHIFT_CONST;