Fix a warning from undefined behaviour sanitizer from RHistogramCnv.h

Fixes

../RootHistCnv/src/RHistogramCnv.h:77:11: runtime error: member call on address 0x0000167703e0 which does not point to an object of type 'TTH'
0x0000167703e0: note: object is of type 'TH1D'
 00 00 00 00  58 7d 77 29 66 7f 00 00  00 00 00 00 08 00 00 03  58 b8 39 40 66 7f 00 00  14 20 31 44
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'TH1D'
../RootHistCnv/src/RHistogramCnv.h:77:11: runtime error: member call on address 0x0000165d3d10 which does not point to an object of type 'TTH'
0x0000165d3d10: note: object is of type 'TH2D'
 00 00 00 00  40 c5 77 29 66 7f 00 00  00 00 00 00 08 00 00 03  58 b8 39 40 66 7f 00 00  14 20 32 44
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'TH2D'
../RootHistCnv/src/RHistogramCnv.h:77:11: runtime error: member call on address 0x000016815c20 which does not point to an object of type 'TTH'
0x000016815c20: note: object is of type 'TH3D'
 00 00 00 00  80 12 78 29 66 7f 00 00  00 00 00 00 08 00 00 03  58 b8 39 40 66 7f 00 00  14 20 33 44
              ^~~~~~~~~~~~~~~~~~~~~~~
              vptr for 'TH3D'