Answer
The decay scheme is $p \longrightarrow e^+ \hspace{2mm}+ \hspace{2mm} \nu_e$
$p$ = proton
$e^+$= positron
$\nu_e$ = electron-neutrino
This does not violate the energy conservation law.
Work Step by Step
It would be sufficient if we go to the rest frame of the proton ($p$) and see if the decay is possible.
Mass of proton = $1.673 \times 10^{-27}$ kg.
Mass of positron = Mass of electron = $9.11 \times 10^{-31}$ kg.
We can take neutrinos to be almost massless. Thus, the combined mass of the positron and electron-neutrino is less than the mass of the proton.
In special relativity, mass is another form of energy. Thus, it is energetically favourable to destroy a proton and create a positron and an electron-neutrino. Thus, the decay scheme does not violate energy conservation.
