Merge pull request #27337 from fengyuentau:4x/build/riscv/fix_warnings

build: fix warnings from recent gcc versions #27337

This PR addresses the following found warnings:
- [x] -Wmaybe-uninitialized
- [x] -Wunused-variable
- [x] -Wsign-compare

Tested building with GCC 14.2 (RISC-V 64).

### Pull Request Readiness Checklist

See details at https://github.com/opencv/opencv/wiki/How_to_contribute#making-a-good-pull-request

- [x] I agree to contribute to the project under Apache 2 License.
- [x] To the best of my knowledge, the proposed patch is not based on a code under GPL or another license that is incompatible with OpenCV
- [x] The PR is proposed to the proper branch
- [ ] There is a reference to the original bug report and related work
- [ ] There is accuracy test, performance test and test data in opencv_extra repository, if applicable
      Patch to opencv_extra has the same branch name.
- [ ] The feature is well documented and sample code can be built with the project CMake

modules/imgproc/src/color_hsv.simd.hpp

@@ -850,7 +850,7 @@ struct RGB2HLS_b
                 for ( ; j <= dn*bufChannels - nBlock*bufChannels;
                       j += nBlock*bufChannels, src += nBlock*4)
                 {
-                    v_uint8 rgb0, rgb1, rgb2, rgb3, dummy;
+                    v_uint8 rgb0, rgb1, rgb2, dummy;
                     v_load_deinterleave(src, rgb0, rgb1, rgb2, dummy);
 
                     v_uint16 d0,d1,d2,d3,d4,d5;

modules/imgproc/src/pyramids.cpp

@@ -769,7 +769,6 @@ template <> int PyrUpVecVOneRow<int, uchar>(int** src, uchar* dst, int width)
             r20 = *(row2 + x);
         int _2r10 = r10 + r10;
         int d = r00 + r20 + (_2r10 + _2r10 + _2r10);
-        int d_shifted = (r10 + r20) << 2;
         // Similar to v_rshr_pack_u<6>(d, vx_setzero_s16()).get0()
         *(dst + x) = (int)((((unsigned int)d) + ((1 << (6 - 1)))) >> 6);
     }